international journal of heat and mass...

International Journal of Heat and Mass Transfer 136 (2019) 911–923

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

Experimental investigation of thermal performance of the oscillatingheat pipe for the grinding wheel

https://doi.org/10.1016/j.ijheatmasstransfer.2019.03.0650017-9310/� 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (Y. Fu).

Ning Qian a, Yucan Fu a,⇑, Yuwen Zhang b, Jiajia Chen c, Jiuhua Xu aaCollege of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, ChinabDepartment of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO 65211, USAcCollege of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, China

a r t i c l e i n f o

Article history:Received 2 February 2019Received in revised form 11 March 2019Accepted 11 March 2019

Keywords:Oscillating heat pipesGrinding wheelTemperature controlThermal resistanceFlow patterns

a b s t r a c t

A massive amount of heat is generated during the high-efficiency grinding process, which leads to a seri-ous burnout problem that limits the increase of the material removal rate. The oscillating heat pipe (OHP)can augment the heat transfer in the grinding zone to avoid the burnout. An oscillating heat pipe grindingwheel that is a combination of a grinding wheel and OHPs was proposed in this paper. Aimed to deter-mine geometric dimensions and operating parameters of the OHP, experimental investigations were car-ried out to study effects of working fluid, inner diameter, and heat flux on the heat transport capability ofOHPs. The dynamics of flow change and coupled effects of geometric dimensions and thermophysicalproperties of working fluids were further analyzed by visualization. The OHP filled with acetone showsan advantage in heat transfer. Within the critical diameter range, a large inner diameter is better forthe thermal performance. As heat flux increases, changes of flow pattern and motion modes from bubblyflow to annular flow and from oscillation to circulation enhance the thermal performance. The innerdiameter of 3 mm and acetone as the working fluid are preferred for better cooling effects.

� 2019 Elsevier Ltd. All rights reserved.

1. Introduction

High-Efficiency Grinding (HEG) technology was developed inthe 1980s, and it is an improvement of high-speed grinding andcreep feed grinding; it is regarded as one of the ‘‘peak” of moderngrinding technology [1]. The HEG is combined with high grindingspeed, fast feed rate, and large depth of cut; therefore, its materialremoval rate can be 100–1000 times higher than that of the con-ventional grinding technology with high ground surface quality,as shown in Fig. 1. However, there remains a burnout phenomenonduring the HEG process. Andrew et al. explained the burnout bythe film boiling theory [2]. When the grinding heat flux is underthe critical heat flux of the coolant, the cooling fluid in the grindingzone sustains the nucleate boiling state. The grinding energy istransferred by the latent heat of phase change; accordingly, thetemperature of the workpiece surface can be maintained under120 �C, which is determined by the critical heat flux. Nevertheless,when grinding difficult-to-machining materials (e.g., titaniumalloy or superalloy), the grinding wheels wore easily due to thehigh strength and low thermal conductivity of the materials. Thegrinding heat flux consequently increases until reaches the critical

heat flux, and then the cooling fluid enters the film boiling regime.In this situation, the vapor film covers the workpiece surface andprohibits the effective heat transfer of the coolant; therefore, theburnout takes place [3–5]. The above research, on one hand, rea-sonably revealed the cause of sudden burnout. On the other hand,the critical heat flux was regarded as the unchangeable physicalproperty and this affected the following development of grindingtechnology.

However, Xu et al. [6] considered that the critical heat fluxcould be improved by augmented heat transfer. In terms of grind-ing, the improvement of critical heat flux means the increase ofmaterial removal rate. Hence, a series of researches on the increaseof critical heat flux were conducted. Zheng and Gao [7] introduceda segmented grinding wheel to improve the cooling condition inthe grinding zone. Sun [8] enhanced the heat transfer by applyingthe radial jet impinging through the grinding wheel. A radial andaxial rotating heat pipe grinding wheel was invented to enhanceheat transfer in the grinding zone through excellent heat transferof heat pipe [9–13]. The Ti6Al4V titanium alloy and Inconel 718superalloy were successfully ground by the heat pipe grindingwheels and the grinding temperature was controlled under150 �C. Although the heat pipe technology is applied in the grind-ing process to enhance the critical heat flux, the increased localpressure caused by rotating limited its application in the

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Nomenclature

Ca capillary numberID (D) inner diameter, mmf frictional factorh heat transfer coefficient, W/m2KK loss coefficientls length of grinding zonen rotating speed, rev/minp pressure, PaQ heating power, Wq00 heat flux, W/m2

R thermal resistance, K/WT temperature, �Cu speed of flow, m/s

Greek symbolsa dynamic contact angle, �l dynamic viscosity, Pa�sq density, kg/m3

r surface tension, N/m

Subscriptsa adiabaticc condensere evaporatoreff effectivein inputout output

Fig. 2. Effect of speed on the plasticity of nickel-based superalloy.

912 N. Qian et al. / International Journal of Heat and Mass Transfer 136 (2019) 911–923

high-speed working condition, which in turn affects the grindingrate. Moreover, the heat transfer performance and the stability ofthe rotating heat pipe will be diminished under high-speed grind-ing conditions. Consequently, another form of heat pipes is calledfor the augment of heat transfer in the grinding zone.

The Oscillating Heat Pipe (OHP) is a novel type of heat pipe,which was first introduced by Akachi in 1990 [14]. The typicalOHP is meandered by a capillary tube that is partially filled withworking fluid [15]. The OHP is divided into 3 different types: closedloop OHP, open loop OHP, and OHP with unidirectional valve;among them the closed loop OHP has a better thermal performance[16]. Many factors influence the thermal performance of OHP suchas the geometric dimension, working fluid, filling ratio (FR), and soon. Khandekar et al. [17] investigated effects of physical propertiesof working fluid and geometrical parameters of OHP on the ther-mal performance. They found that the working fluid and diameterof the tube have the coupled influence, especially when the innerdiameter of the tube is 2 mm; the thermal resistance of OHP filledwith water is lower than that filled with R123 and ethanol. Whenthe inner diameter of the tube is 1 mm, the thermal resistance ofOHP filled with water is the largest. When charging with water,for inner diameter of 1.2, 2, and 2.4 mm, the best filling ratio is50%, 40%, and 40%, respectively; while the best filling ratio is 40%when filled with ethanol. Ma et al. [5,18–21] found that the fre-quency of the oscillation is relative with the latent heat through

Fig. 1. Schematic of grinding (a) the axial rotating grin

both experiment and simulation. Moreover, the amplitude of theoscillation is affected by the specific heat, while the most heat istransferred by the sensible heat of the working fluid through theOHP. Zhang et al. [22] studied the oscillating characteristics ofOHPs charged with FC21, ethanol, and water, they shared resembleresults with Ma et al. Zhang and Faghri [23] concluded that the sur-

ding wheel (b) the radial rotating grinding wheel.

N. Qian et al. / International Journal of Heat and Mass Transfer 136 (2019) 911–923 913

face tension affects the formation of vapor bubbles and liquid plugsbut the surface tension rarely influents the thermal performance.Liu et al. [24] investigated start-up characteristics of the OHP,and found that fluid with low dynamic viscous, low specific heat,and high saturation pressure gradient had a good start-up perfor-mance. Khanderkar et al. [25] found that the filling ratio range ofOHP is 25–65%, below which local dry-out might occur and dimin-ish the overall heat transfer. When the filling ration is above thisrange, the fluid has difficulty in effective oscillating motion, andthe thermal performance is therefore limited.

The previous researches show that the OHP has a high heattransfer ability, simple structure and unique motion of workingfluid. Therefore, the OHP has the possibility to overcome the men-tioned limitation of applying heat pipe into the grinding processand to combine with the grinding wheel. The thermal performanceof the OHP directly influences the heat transfer ability of the OHPfor the grinding wheel to augment heat transfer. Many experimen-tal, theoretical and simulation investigations of the thermal perfor-mance of OHPs were conducted in the past. However, in terms ofintegrating the OHP with grinding wheel in the grinding process,the thermal performance and operating conditions of OHPs arenot reported before and in demand of further study.

Aimed to determine the geometric dimensions and operatingparameters of the OHP, which is applied in the grinding wheel, thiswork focuses on the thermal performance of the OHP. Specifically,a static experiment was designed, although the OHP grindingwheel rotates during the grinding. In the grinding process, basedon the grinding principle, when grinding the hard-to-machiningmaterials such as nickel-based superalloy and titanium alloy, thegrindability is related to the deformation speed of the material.The plasticity of the material is shown in Fig. 2. When increasingthe grinding speed, the deformation increased accordingly. Interms of the high-efficiency grinding, the material is in the b-c-dregion in the Fig. 2, i.e., in the thermal effect regime. When increas-ing the grinding speed, the plasticity of the material rises, corre-spondingly, the chip formation becomes easier, grinding forcereduces, and the heat generated in the grinding zone decreases.Therefore, if the OHP in the static state can sucessfully transferthe specific heat, it will work better under the rotation condition.

An experimental investigation was conducted to explore theinfluence of the inner diameter of the tube, working fluids and heatflux on the heat transfer ability. In this article, the concept of oscil-lating heat pipe grinding wheel is introduced in Section 2. Section 3includes the experimental setup and methods of data process.Effects of working fluids, dimensions and heat flux on the heattransfer capacity are involved in Section 4. The development offlows is studies through visualization in Section 5. Moreover, thecoupled effects of inner diameter and working fluids is discussedin Section 5. At last, main conclusions are drawn in Section 6.

2. Concept of oscillating heat pipe grinding wheel

Considering the high heat transfer ability of OHPs, an idea ofapplying the OHP into grinding wheel to augment heat transferinside the grinding zone is introduced in this paper. The oscillatingheat pipe grinding wheel is produced by manufacturing the OHPchannels inside the matrix of a grinding wheel, as shown inFig. 3. When applying the OHP grinding wheel into the machiningprocess, the grinding heat will transfer from the grinding zone tothe outside through the working layer of the grinding wheel, evap-orator, adiabatic, and condenser sections of the OHP. Hence, thegrinding heat is forced to transfer out to avoid high grinding tem-perature, workpiece burnout, and fast grinding wheel wear.Besides, with the assistance of the high heat transport capacity ofOHPs, the harmful cooling fluid can be reduced or even eliminated

during the process. As a result, a green and environmentallyfriendly grinding process can be achieved.

The heat flux generated in the grinding zone is around1 � 106 W/m2 [2–4], which is huge for both conventional liquidcoolant and the OHP. However, the grinding process is a discontin-uous process. When the OHP is in the grinding zone, the evaporatorof the OHP is subjected to a pulsating thermal load. When the OHProtates outside the grinding zone, there is no heat transferred intothe evaporator. In terms of each point on the evaporator of theOHP, the pulsating thermal load can be replaced by the effectiveaverage heat load, which has the same thermal effect with the pul-sating thermal load. Although the peak value of the pulsating heatflux is as high as 1 � 106 W/m2, the duty ratio of time is very low,i.e., 0.5�2%. As a result, the effective average heat flux alters from5000W/m2 to 20,000W/m2. The OHP can perfectly operate undersuch heat flux range according to previous researches [15,16]. As aresult, the OHP has the potential to be applied to the grindingwheel to enhance the heat transfer.

3. Experimental setup and data processing

3.1. Experimental setup

For simplification, the OHP channelwas extracted from the oscil-lating heat pipe grindingwheel, as illustrated in Fig. 3. A single-loopOHP, which can be considered as the basic constituent of an OHP,was used in this work to investigate the thermal characteristics.

The experimental setup is shown in Fig. 4. There are two kindsof OHPs, copper OHP and glass OHP (for visualization). The OHP is120 ± 2 mm long and 30 ± 2 mm wide, which is suitable for thegrinding wheel. The critical inner diameters of the OHP filled withacetone, ethanol and DI water are around 3 mm, 3 mm, and 5 mm,respectively [26]. Hence, the inner diameters (ID) and outer diam-eters (OD) of the OHP in this work are 0.8, 2, 3 mm and 2, 3, 5 mm,respectively. The thermal load was provided by KIKUSUI PZB40-10power supply and Ni-Cr heating wire (limits of error: ±0.1 W) andthe OHP was bottom-heated. The heat flux ranges from 4550 to22,750 W/m2, which covers the real grinding heat flux as men-tioned in Section 2. The OHP was cooled air with a pressure of0.6 MPa and temperature of 0–5 �C. The evaporator and adiabaticsection were covered by the insulation. The temperature of evapo-rator, adiabatic section, and condenser were measured by theOmega K-type thermocouples (temperature range from �267 �Cto 260 �C with error of 1.1 �C or 0.4%); the data were acquired byNI-USB6366 card (at 20 Hz DAQ frequency) and processed by NILabView and NI DIAdem software. The environment temperaturewas maintained at 20 ± 1 �C. The flow of working fluid inside theOHP was captured by Sony AX700 camera at the frame speed of1000 fps under the super slow-motion mode. The detailed experi-mental conditions are listed in Table 1.

Three kinds of liquid were applied in the experiment as workingfluid: acetone (CH3COCH3), deionized (DI) water, and ethanol(C2H5OH). The filling ratio (FR) is defined as the ratio of volumeof working fluid filled in and the volume of inside space, the fillingratio for all cases was 55%. The thermophysical properties of theworking fluid are tabulated in Table 2. The DI water has the highestboiling point, density, specific heat, thermal conductivity, latentheat, and surface tension. Acetone and ethanol have lower density,specific heat, latent heat, and surface tension, while ethanol hasthe highest dynamic viscosity.

3.2. Data processing

The whole system is allowed to run until the steady state wasreached. The desired heat power is monitored through voltmeter

Fig. 3. Schematic of the oscillating heat pipe grinding wheel (a) axial rotating (b) radial rotating.


and ammeter. The temperature of the evaporator, adiabatic sec-tion, and condenser on each tube are measured. Temperatures ofevaporator, adiabatic and condenser sections, i.e., Te, Ta, Tc, arethe time-averaged temperatures of the corresponding measuredtemperatures.

The thermal performance of OHP was calculated based on theaverage evaporator and condenser temperature. The overall ther-mal resistance is obtained from:

Rtotal ¼ DTQinð1Þ

The structure thermal resistance is:

Rstructure ¼ DTQinðFR ¼ 0Þ ð2Þ

The overall thermal resistance can be considered as a parallelconnection of thermal resistance of the structure and the effectivethermal resistance of OHP [27] (see Fig. 5):

ROHP�eff ¼ 1Rtotal �1

Rstructure

� ��1ð3Þ

The uncertainties of the indirect measurement parameters wereobtained based on error propagation principles as reported by pre-vious work [28–30] is shown in Table 3.

4. Experimental results

4.1. Effects of working fluid on the thermal performance

Acetone, DI water, and ethanol have different thermophysicalproperties. Therefore, when acting as working fluids inside theOHP, they have different responses to the thermal load. It is neces-sary to investigate the influence of working fluid on the thermalperformance, i.e., temperature control and thermal resistance. Inorder to select the working fluid of the OHP for the grinding wheel,the thermal performance of OHPs filled with these three fluids areevaluated.

Figure 6(a) shows the temperature of OHPs filled with acetone,DI water, and ethanol. For the inner diameter of 3 mm, the similartendency of the evaporator temperature was found when the OHPfilled with acetone and ethanol. When the heat flux rises from4550W/m2 to 22,750 W/m2, the evaporator temperatureincreases gradually and peaks at the heat flux of 9100W/m2

and 13,650 W/m2, respectively. Whereas, the evaporator temper-ature of the OHP filled with DI water increases linearly. When theheat flux is under 14,000 W/m2, the OHP filled with DI water hasthe lowest evaporator temperature of 44.21 �C. On the otherhand, when the heat flux increases from 14,000 W/m2 to22,750W/m2, the evaporator temperature of OHP filled with ace-tone is the lowest at 60.88 �C. Nevertheless, with increasing heat

Fig. 4. The schematic of experimental setup.

Table 1The experiment conditions.

Equipment Type

Power KIKUSUI PZB40-10 powerHeating U0.1 mm Ni-Cr heating wireAir coolant 5–15 �C, 0.6 MPa cold airThermal couple Omega K-typeAcquisition NI-USB6366 card (20 Hz)Camera Sony AX700 1000fps super slow motionInner diameter (mm) 0.8, 2, 3Heat flux (W/m2) 4550, 9100, 13,650, 18,200, 22,750


flux, all condenser temperature of OHPs filled with acetone, DIwater, and ethanol increase linearly from around 21 �C by141.2%, 120.6%, and 112.4%, respectively. Clearly, the condensertemperature of OHP filled with acetone is always the highest withthe change of heat flux. In general, it shows that different work-ing fluids have different temperature control performances. Whenheat flux is above 14,000W/m2, the acetone has the best temper-ature control ability, followed by the ethanol and then DI water.

Table 2The physical properties of working fluid at 1 atm.

Fluid Boilingpoint (�C)

Density(kg/m3)

Specific heat(kJ/kg��C)

Thermal conductivity(W/m�K)

L(

DI water 100.0 998 4.18 0.599 2Acetone 56.2 792 2.35 0.170 5Ethanol 78.3 789 2.39 0.172 8

However, when heat flux is under 14,000 W/m2, DI water hasthe best temperature control ability.

Relatively, the thermal resistance of the OHPs filled with DIwater decreases gradually, as illustrated in Fig. 6(b). The thermalresistance of OHPs filled with acetone and ethanol drops slowlyand then decreases dramatically when the heat flux exceeds14,000 W/m2. When the heat flux is under 14,000 W/m2, DI waterhas the best thermal performance: the thermal resistance is about74% lower than that of acetone and ethanol. Whereas the acetonehas the best thermal performance when the heat flux is above14,000 W/m2: the thermal resistance is slightly lower than thatof DI water and ethanol.

Figure 7 shows the effects of working fluids on the temperatureand resistance of OHP with ID = 2 mm. It can be seen that, whenthe inner diameter is 2 mm, the temperatures of OHPs filled withacetone, DI water and ethanol rise gradually with increasing heatflux. Moreover, when the heat flux is 22,750 W/m2, the evaporatortemperature of the OHP filled with DI water exceeds 120 �C, whichis known as the critical temperature for the grinding zone markedby the shadow area [2,3]. Besides, the thermal resistances of OHPsfilled with three working fluids descend as heat flux increases. It is

atent heatkJ/kg)

(dp/dT)sat at 60–80 �C(�103 Pa/�C)

Dynamic viscosity(mPa�s)

Surface tension(mN/m)

257 1.30 1.01 72.823 3.10 0.32 23.746 3.51 1.15 22.8

Fig. 5. Thermal resistance model of OHP.

Table 3Maximum uncertainties of main parameters.

Parameters Te Tc Q ROHP-eff

Maximum uncertainties (%) 0.4 0.4 1.0 1.1

Fig. 6. The effect of working fluid on (a) the temperature and (b) the thermalresistance of OHP (ID = 3 mm).

Fig. 7. Effects of working fluid on (a) the temperature, (b) the thermal resistance ofOHP (ID = 2 mm).


clearly illustrated in Fig. 7 that the OHP filled with acetone has thebest thermal performance followed by the OHP filled with ethanol.The OHP filled with DI water, on the contrary, has the worst ther-mal performance.

Figure 8 shows the effect of working fluid on the thermal per-formance when the inner diameter is 0.8 mm. The temperature

of OHPs increases steadily, except for the condenser temperatureof OHP filled with DI water. The condenser temperature of OHPfilled with DI water remains stable around 20 �C, indicating thatthere is no oscillation. In addition, the evaporator temperature ofOHP filled with acetone exceeds that of OHP filled with ethanolgradually, along with the increase of the heat flux, as shown inFig. 8(a). Similar to the temperature, the thermal resistance ofthe OHP filled with DI water remain stable around 9 K/W. Never-theless, as the heat flux increases, the thermal resistance of theOHPs filled with acetone and ethanol drops sharply by 43.2% and48.6%, then stays stable around 6.3 K/W and 5.6 K/W, respectively.Besides, the thermal resistance of the OHP filled with ethanolbecomes lower than that of the OHP filled with acetone whenthe heat flux is over 14,000 W/m2.

From the above results, the ethanol has the best thermal perfor-mance when the heat flux is under 14,000 W/m2, while the heatflux is above 14,000W/m2, the acetone has the best thermal per-formance, regardless of the inner diameter. In terms of the grindingprocess, increase of material removal rate means a higher heat flux.Therefore, in order to have a better augmented heat transfer in thegrinding zone when rising the material removal rate, the acetone isrecommended as the working fluid.

4.2. Effects of inner diameters on the thermal performance

Generally, the volume and the radius of the grinding wheel arelimited. Therefore, the dimension of the OHP inside the grindingwheel should be optimized for a better thermal performance. Inaddition, the surface tension and gravity have different impacts

Fig. 8. The effect of working fluid on (a) the evaporator and condenser temperatureand (b) the thermal resistance of OHP (ID = 0.8 mm).

Fig. 9. The evaporator and condenser temperature of the OHP filled with acetone.

Fig. 10. The thermal resistance of the OHP filled with acetone.


on the operating mechanism of the working fluid under differentinner diameters. As a result, it is important to analyze the effectof the tube inner diameter on the thermal performance.

From the experimental results, the effect of inner diameter onthe thermal performance of the OHP filled with acetone is typical.Therefore, the acetone as working fluid is focused on this section.The evaporator temperature exhibits a rising trend with increasingheat flux. When the inner diameter is 3 mm, the evaporator tem-perature rises firstly from 38.17 �C to 59.25 �C, then drops to49.33 �C at a heat flux of 14,000 W/m2, and finally increases slowlyto 60.88 �C. When the inner diameters are 2 mm and 0.8 mm, theevaporator temperatures increase almost linearly with the increaseof heat flux. When the heat flux is under 14,000 W/m2, the evapo-rator temperature of ID = 3 mm is the highest. However, when theheat flux is above 14,000 W/m2, the evaporator temperature ofID = 0.8 mm is the highest, followed by that of ID = 2 mm; theevaporator temperature of ID = 3 mm is the lowest. The condensertemperatures of all three inner diameters rise gradually with theincrease of heat flux. The condenser temperature of ID = 3 mm isthe highest, i.e., 50.65 �C, at the heat flux of 22,750 W/m2, followedby the condenser temperature of ID = 0.8 mm, i.e., 38.20 �C, and thecondenser temperature of ID = 2 mm, i.e., 35.64 �C. The tempera-ture difference between the evaporator and the condenser ofID = 3 mm is a humped curve: the highest value is 30.09 �C at theheat flux of 9100 W/m2, while the lowest value is 13.58 �C at theheat flux of 22,750 W/m2. Hoverer, the temperature difference ofID = 2 mm and ID = 0.8 mm increases vigorously from around22 �C to 59.18 �C and 79.99 �C, respectively. It can be concludedthat the OHP of ID = 3 mm has the best temperature control ability

and temperature uniformity, followed by the OHP of ID = 2 mmand the OHP of ID = 0.8 mm, as shown in Fig. 9.

In terms of the thermal resistance, it is clear from Fig. 10 thatthe thermal resistance of OHPs decrease with increasing heat flux.The OHP of ID = 3 mm shows the lowest thermal resistance of0.54 K/W which is 91% and 81% lower than the OHP of ID = 0.8 mmand ID = 2 mm at a heat flux of 22,750 W/m2.

The inner diameter of OHPs filled with ethanol and DI water hasthe same effect on the thermal performance with the OHPs filledwith acetone. The effect of inner diameter of the OHP on the ther-mal performance is caused by the heat capacity of the OHP withdifferent inner diameter, which leads to different volume of theworking fluid. The OHPs with larger inner diameter, which is stilllower than the critical diameter, has a higher heat capacity andbetter thermal performance. Given that, within the critical diame-ter, a larger diameter is preferred. The grinding wheel, as illus-trated in Fig. 3, consists of several single loop OHPs, which aremade of long straight holes and grooves in the wheel body. A largerdiameter will make the manufacturing of OHPs easier. In this case,the diameter of 3 mm is recommended. Moreover, the effect of theinner diameter is the result of the competition between the capil-lary effect that impede the oscillation, and the pressure differencethat drives the oscillation. This will be further analyzed in Section 5in detail.

Fig. 11. The flow pattern in the OHP (vapor bubbles are filled in red). (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)


4.3. Effects of heat flux on the thermal performance

From the experimental results displayed above, it illustratesthat the heat flux also affects the thermal performance, especiallyas for the working fluid of acetone and ethanol. Therefore, theeffect of the heat flux is analyzed in this section.

There is a turning point of heat flux for the thermal performanceof OHPs filled with acetone and ethanol. When the heat flux isunder 14,000 W/m2, it is defined as the low heat flux. Otherwise,the heat flux is referred to as the high heat flux. In terms of thetemperature of the OHP of ID = 3 mm, the evaporator temperaturecurves with acetone and ethanol are hump-shaped curves. Theevaporator temperature increases rapidly at the low heat fluxregion and reaches the peak point near the critical heat flux. After-ward, the evaporator temperature decreases and then rises slowlyat the high heat flux region, as illustrated in Fig. 6(a). The thermalresistance of OHPs filled with acetone and ethanol are S-shapedalong with the heat flux. At the low and high heat flux region,the thermal resistance decreases tardily. While the heat flux is nearthe critical heat flux, the thermal resistance decreases sharply, asillustrated in Fig. 6(b).

Similar to the situation of the OHP of ID = 3 mm, there remainsthe same turning point of heat flux, i.e., 14,000 W/m2 for the ther-mal performance of the OHP of ID = 0.8 mm, especially for theworking fluid of acetone and ethanol. As for the temperature, theevaporator temperature of the OHPs filledwith acetone and ethanolare S-shaped curves, which turning point is at the critical heat flux,i.e., 14,000 W/m2. In addition, the thermal resistance of the OHPfilled with acetone and ethanol decreases sharply from 11 K/W to6 K/W and 5.6 K/W, respectively at the low heat flux region, andthen the thermal resistance stays steady, as illustrated in Fig. 8.

At high heat flux, OHPs show a better thermal performance,which is benefit for the heat transfer during the grinding process.As the material removal rate increases, the grinding heat flux getshigher. In turn, the heat transfer capacity of OHPs increase and theaugmented heat transfer in the grinding zone provided by the OHPwill be effective.

5. Flow patterns

5.1. Evolutions of flow pattern and motion mode

From the experimental results mentioned in Section 4.3, thethermal performance has a strong connection with the heat flux.Through the visualized experiments, the increase of heat fluxaffects the thermal performance via the changes of flow patternand motion mode of the working fluid. Fig. 11 shows typical flowpatterns in an OHP. With increasing heat flux, the working fluidexperiences six periods: interface evaporation, bubbly flow, vaporplug flow, annular flow, post dry-out, and vapor forced convection.When the heat flux is low and the temperature is below the satu-ration temperature, there is no bubble generating in the evapora-tor; therefore, superheated liquid rises to the liquid-vaporinterface via single-phase convection where the evaporation takesplace. During this period, the heat transfer coefficient is constant.As the heat flux and temperature rise, the generation of bubblesoccurs, marking the start-up of the OHP. Correspondingly, the heattransfer coefficient increases gradually with the development ofthe bubbly flow. With increasing heat load, the rising temperatureand pressure drive bubbles rising faster to merge. The merge ofbubbles fuses into vapor plugs. As a result, the flow patternbecomes the vapor plug flow, in this period the heat transfer coef-ficient remains stable. As the vapor plugs continue to grow, the liq-uid slugs between the plugs evaporate gradually. Then the vaporplugs fuse into the annular flow. As for the annular flow, the thin-

ner the liquid film gets, the higher the heat transfer coefficient is.At the moment that liquid film completely turns into vapor, andit reaches the dry-out state and the heat transfer coefficient dropssharply at the dry-out point. Therefore, with the increase of heatload, the flow of working fluid develops from interface evaporationto the annular flow in the evaporator, and under the severe ther-mal condition, the liquid dries out. The changes of flow patterndetermine the thermal transfer ability in one respect, especiallythe ability to transfer heat into the OHP from the heat source.

Besides the flow pattern, the increase of heat flux leads to thechanges of flow motion of the working fluid, which has an impacton the thermal performance of the OHP. Fig. 12 illustrates the pho-tographs of OHPs from visualized experiments. At the heat flux of4,550W/m2, the working fluid in the evaporator is in the quasi-static state; part of the heat is transferred out through the OHPtube, and another part is absorbed and stored in the still workingfluid. Therefore, the thermal performance is limited. When the heatflux rises to 9,100W/m2, the oscillation starts up and the pendu-lum oscillation is the main motion mode of the working fluid.There is no bubble generation in the pendulum oscillation. The ini-tial bubbles or vapor plugs on one side of the tube near evaporatorare heated and expanded to the condenser, which is shown as thevapor plug marked by red arrows in Fig. 12(b) I. It drives the bub-bles or vapor plugs in the other side of the tube moving towardsthe evaporator, which shown as the vapor plug marked by bluearrows in Fig. 12(b) I. When this vapor plug moves into the evap-orator, the heated vapor plug expands to the condenser and drivesother vapor plugs moving back to the evaporator, as illustrated inFig. 12(b) II. The process loops in this pattern as the pendulumoscillation. The expanded vapor plug moves slower and slower assoon as it exits the evaporator and changes the direction beforecompletely entering the condenser. Consequently, the heatexchange between the evaporator and condenser is restricted,

Fig. 12. The flow pattern and motion mode of the OHP (vapor bubbles are filled in red). (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)


and the thermal performance is still relatively low. As the heat fluxincreases to 13,650 W/m2, the newly growing bubbles and vaporplugs move through the condenser and condense there, then disap-

pear or shrink into small bubbles in the tube bend or another sideof the tube of the OHP, as marked by black arrows in Fig. 12(c) I andII. The large pressure difference caused by higher temperature and

Fig. 13. Force analysis of a liquid plug moving in the tube in experiments (a)moving upwards and (b) moving downwards.

Fig. 14. The bubble velocity of working fluid inside the OHP.


heat flux drives the working fluid circulating in the OHP. Themotion from the evaporator to the condenser of the fluid, whichis referred as the transition of oscillation and circulation, benefitsthe heat transfer out of the OHP through the condenser; thedecrease of the evaporator temperature is another sign of thisimproved heat exchange. Thereby, the thermal performanceenhances observably.

In addition, there exist bubble formations in the evaporator dueto high heat flux, as illustrated in Fig. 12(c) III and IV. The circula-tion direction can alter from time to time under such condition.When the heat flux of 18,200W/m2 is applied, the motion developsinto circulation flow, though there are direction changes. When theliquid flows into the evaporator, a series of bubbles generate,expand, and merge into vapor plugs and annular flows, as illus-trated in Fig. 12(d) I. In terms of the dynamics of direction change,as the vapor plug enters the evaporator, marked by the black arrowin Fig. 12(d) I, the vapor plug expands quickly to both sides, andgradually achieves the upward moving trend, marked by the blackarrow in Fig. 12(d) II. Then the overall flow motion stops andchanges the direction, while the vapor plug flowing upwards, asshown in Fig. 12(c) III. Accordingly, along with the developmentof flowmotion into circulation, the thermal performance continuesto improve. When the heat flux reaches 22,750 W/m2, it turns intostable unidirectional circulating motion with vapor plug flow andannular flow, as illustrated in Fig. 12(e). Once the heat goes intothe evaporator, it can be carried to the condenser by the steady cir-culating flow of working fluid and then transferred out of the OHPin the condenser. Due to the stable unidirectional flow, the heatload can be transferred timely; thus, the thermal performancekeeps enhancing.

In conclusion, the efficiency of heat transfer from the evaporatorto the condenser, which involves the quantity of heat and the trans-fer rate, depends on the motion mode of the working fluid. Underdifferent heat fluxes, the relative flow pattern combined with themotion mode of the working fluid influence the thermal perfor-mance of the OHP, i.e., the temperature control ability and the ther-mal resistance. Specifically, when the heat flux increases, thethermal performance improves along with the flow pattern alteringfrom interface evaporation to vapor plug flow, then to annular flow,as well as the motion mode changing from the quasi-static state topendulum oscillation, then to circulation. However, under severeheating conditions, the thermal performance deteriorates due tothe temporary local dry-out in the evaporator.

5.2. The coupled effect of the inner diameter and working fluid

The inner diameter and the thermophysical property of theworking fluid have coupled influences on the flow motion andthe thermal performance. As shown in Fig. 13, when the fluid ismoving upwards, one driving force caused by the temperature dif-ference and four resistances, i.e., gravity, capillary force, frictionand pressure drop from the bend meandering, act on the liquidplug. When the fluid flowing downwards, the pressure caused bythe temperature difference and gravity are driving forces whilethe capillary force, friction and pressure drop from the bend mean-dering are resistances.

It is assumed that all liquid plugs behave as one liquid plug andall vapor bubbles as one vapor bubble for simplification. The pres-sure difference caused by the evaporator and condenser tempera-ture difference drives the liquid plug moving upwards andrestricts the plug flowing downwards. The pressure caused bythe temperature difference, which is applied on the liquid plug,can be calculated by:

Dptemp ¼@p@T

� �sat

DT ð4Þ

Due to the capillary force in the tube, the motion encounters anadditional resistance, which is referred to as capillary resistance,according to the dynamic contact angle hypothesis [30,31]. Byapplying the Laplace-Young equation for capillary pressure gradi-ent across a liquid-vapor interface, the capillary resistance can bedetermined by:

Dpcapillary ¼ 32aReCa

� �0:33 luD

ð5Þ

where a is a constant equal to 0.17, the D is the inner diameter ofthe tube, Ca is the capillary number, l is the dynamic viscosityand Re is the Reynolds number.

As the working fluid flows through the tube, the frictional forcearises from the interaction between the liquid/vapor and the tubewalls, which can be evaluated by [15]:

Dpfriction ¼ fqLeff u2

2Dð6Þ

where q is the density, and u is the flow velocity. Leff is the effectivelength of the OHP, which is defined as Leff = La + (Le + Lc)/2. For alaminar flow in a round tube, the frictional factor, f, can beexpressed as:

f ¼ 64Re

ð7Þ

Substituting Eq. (7) into Eq. (6), the frictional resistance can beobtained as:

Dpfriction ¼32Lefflu

D2ð8Þ


For the unit length of a liquid plug, the gravity can be deter-mined by:

Dpgravity ¼ qg ð9Þ

Fig. 15. Photographs of backflow in the evaporator (ID = 2 mm OHP filled withacetone).

Fig. 16. The flow motion maps of (a) the OHP filled with acetone, (b

For the pressure drop from the tube bend meandering, it wascalculated in terms of the 3-K method [31]:

Dpbend ¼K2qu2 ð10Þ

where the K is the loss coefficient.Therefore, considering Eqs. (4), (5), (8)–(10), the total pressure

difference when the liquid plug flowing upwards and downwardscan be expressed as:

Dp ¼ @p@T

� �satDT � qg � 32a Re

Ca

� �0:33� luD

þ 32LeffluD2

þ K2qu2

" #

ð11ÞExamining Eq. (11), the coupled effects of the inner diameter

and the thermophysical property on the flow motion is obvious.A large (op/oT)sat is beneficial to the flow motion from the evapora-tor to condenser. On the contrary, large surface tension anddynamic viscosity are harmful to the thermal performance due tothe higher resistance. In addition, the smaller the inner diameteris, the higher the capillary resistance is. Consequently, the resis-tance caused by the smaller inner diameter deters the flow motionand deteriorates the thermal performance.

Specifically, the (op/oT)sat of acetone, ethanol and DI water are3.10, 3.51 and 1.30 kPa �C�1, respectively. Hence, the driving forceof acetone and ethanol are twice higher than that of DI water.Moreover, the surface tension of acetone, DI water and ethanolare 22.8, 23.7 and 72.8 mN/m, the dynamic viscosity of acetone,DI water and ethanol are 0.32, 1.01 and 1.15 mPa s and the densityof acetone, DI water and ethanol are 792, 789 and 998 kg/m3,respectively. As a result, the flow resistance of the OHPs filled withacetone and ethanol is significantly lower than that of the OHPfilled with DI water. In addition, according to Eq. (11), as for theOHPs filled with same working fluid, the smaller inner diameter

) the OHP filled with DI water, (c) the OHP filled with ethanol.


leads to higher resistance, which can deteriorate the thermal per-formance. The flow velocity also reveals the intensity of the flowmotion. As shown in Fig. 14, when inner diameter is 3 mm, theflow velocity of the OHP filled with acetone is more than twiceand three times of that of the OHP filled with ethanol and DI water.When the working fluid is acetone, the flow speed of the ID = 3 mmOHP is almost twice of that of the ID = 2 mm OHP.

There is backflow observed during experiments. Figure 15shows the photographs of backflow in the experiments, undersome occasions, the evaporator is full of vapor, except for the thinliquid film on the wall of the tube. However, owing to the lowcapillary resistance and surface tension, some liquid will flowback to the evaporator with help of gravity and gather gradually.Finally, it turns into a liquid plug. The backflow avoids the localdry-out in the evaporator and protects the thermal performancefrom deteriorating too much, especially under high heat flux con-ditions. Due to the small surface tension and capillary resistance,the OHP filled with acetone and ethanol has the backflow regard-less of the inner diameter. The OHP filled with DI water has thebackflow when the inner diameter is above 2 mm. Owning tothe large capillary force, there is no backflow occurring for theOHP filled with DI water when the inner diameter is smaller2 mm.

The flow mode across the whole range of operating conditionsis summarized in the flow motion maps shown in Fig. 16. Theseindicate the influence of heat flux and the coupled effects of theinner diameter and physical property. Clearly, the unidirectionalflow is recognized to have the best thermal performance. Thechanges of flow from oscillation to circulation lead to an improve-ment of thermal performance due to the augment of heat exchangebetween evaporator and condenser. In general, the circulation withplug and annular flow is preferred in the operation. The capillaryforce caused by the small diameter is harmful for the flow motion;consequently, the heat transfer capacity is limited. Besides, largedynamic viscosity and surface tension also resist the motion ofworking fluid. It is clear in Fig. 16 that there is no circulation whenthe inner diameter is 0.8 mm for all three kinds of working fluid.The temporary local dry-out happens when the inner diameter issmaller than 2 mm for DI water, regardless of the heat flux. There-fore, in order to have a better thermal performance, the innerdiameter of 3 mm and the working fluid of acetone arerecommended.

6. Conclusion

A concept of oscillating heat pipe grinding wheel is introducedin this paper to augment heat transfer inside the grinding zone andto avoid the burnout during the process. A preliminary investiga-tion for a single-loop OHP which is the basic constituent of theOHP was carried out. Measurements of temperatures and visualobservations were conducted to investigate the effects of workingfluid, inner diameter and heat flux on the thermal performance.Main conclusions are drawn as follows:

1. Due to the coupled effects of capillary resistance causedby a small diameter and thermophysical property such as(op/oT)sat, dynamic viscosity and surface tension, the OHP filledwith ethanol has a better thermal performance at low heat flux,while the OHP filled with acetone has the best thermal perfor-mance at high heat flux.

2. Along with the decrease of inner diameter, the resistance ofoscillation and circulation motion increase, which deterioratesthe heat transfer. Besides, the smaller diameter is harmful tothe backflow and adds the chance of the temporary local dry-out in the evaporator, which adversely affects the thermalperformance.

3. There exists a turning point of thermal performance at the heatflux of 14,000 W/m2. Through the visualization, at the heat fluxof 14,000 W/m2, the flow changes from bubbly flow to plug flowand annular flow gradually; the motion changes from oscilla-tion to circulation. On one hand, the change of flow patternenhances the ability to absorb heat from the heat source. Onthe other hand, the change of flow motion improves the heattransfer from the evaporator to the condenser. Thereby, moreheat will transfer out of the OHP instead of storing inside theOHP in the form of temperature rise.

In summary, in order to have the best temperature control, theheat transfer for the OHP under the grinding heat flux range, theworking fluid should operate in the unidirectional circulationregime with plug and annular flow. Therefore, the inner diameterof 3 mm and the acetone as working fluid are preferred. Moreover,the effect of rotation on the heat transfer performance will be stud-ied in the future. Accordingly, the oscillating heat pipe grindingwheel will be designed, fabricated, and tested. Due to the highthermal transfer ability of OHPs, the cooling fluid can be reducedin the grinding process, and the oscillating heat pipe will providea high-efficient and green method for the industry.

Declaration of interest

There are no conflicts of interest.

Acknowledgment

The authors gratefully acknowledge the financial support forthis work by the National Natural Science Foundation of China(No. 51175254).

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Experimental investigation of thermal performance of the oscillating heat pipe for the grinding wheel1 Introduction2 Concept of oscillating heat pipe grinding wheel3 Experimental setup and data processing3.1 Experimental setup3.2 Data processing

4 Experimental results4.1 Effects of working fluid on the thermal performance4.2 Effects of inner diameters on the thermal performance4.3 Effects of heat flux on the thermal performance

5 Flow patterns5.1 Evolutions of flow pattern and motion mode5.2 The coupled effect of the inner diameter and working fluid

6 ConclusionDeclaration of interestAcknowledgmentReferences

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