The Effect of Working Fluid Inventory on the Performance of Revolving Helically-Grooved

Heat Pipes

Presenter:

Dr. Scott K. Thomas, Wright State University

Co-authors:

R. Michael Castle, Graduate Research Assistant

(Currently with Belcan Corp.)

Dr. Kirk L. Yerkes, AFRL/PRPG

Objectives

• Determine

– Capillary Limit

– Thermal Resistance

– Evaporative Heat Transfer Coefficient

• Vary

– Heat Input

– Radial Acceleration

– Fluid Inventory

Applications of Revolving Heat Pipes

• Thermal Management of Rotating Devices– Aircraft Generators

– Large-Scale Industrial Electric Motors

– Rotating Satellites

Curved Heat PipeStraight Heat Pipe

R R ω

ω

Previous ResearchKlasing, K., Thomas, S., and Yerkes, K., 1999, “Prediction of the Operating Limits of Revolving

Helically-Grooved Heat Pipes,” ASME Journal of Heat Transfer, Vol. 121, pp. 213-217.

Thomas, S., Klasing, K., and Yerkes, K., 1998, “The Effects of Transverse Acceleration-Induced Body Forces on the Capillary Limit of Helically-Grooved Heat Pipes,” ASME Journal of Heat Transfer, Vol. 120, pp. 441-451.

Findings:

• Capillary limit increased significantly with radial acceleration

• Straight axial grooves showed no improvement with radial acceleration

Shortcomings:

• Effect of liquid fill not examined

• Helical groove geometry not rigorously determined

Working Fluid Inventory

mt = mv + ml = Vvs/vv + GVgr/vl Total Inventory Mass

Vvs = πDvs2Lt/4 + Vgr(1 - G) Vapor Space Volume

Vgr = LgrNgrAgr Groove Volume

Agr = wh + h2(tan θ1 + tan θ2) /2 Groove Area

Lgr = Lt[(2πrh/p)2 + 1]1/2 Groove Length

p = 2π(s - s1)/(φ - φ1) Groove Pitch

G = Vl/Vg Ratio of Liquid Volume to Total Groove Volume

Working Fluid Inventory

• Measure groove height and width– Bitmap image from

microscope

– Microscope scale

– Adobe Illustrator

w

h

θ1 θ2

Working Fluid Inventory

• Measure helical groove pitch– Angular transducer

– High precision voltmeter

– Vertical milling machine

Working Fluid Inventory

θ1

hw

θ2

V - V1

s - s1

Dvs

Lt

Agr

prh

Lgr

Vgr

0.03831 + - 0.00076 cm0.03445 + - 0.0010 cm15.44 + - 0.91 arc deg13.80 + - 0.96 arc deg1.474 + - 0.064 Volt10.000 + - 0.023 cm1.359 + - 0.005 cm43.8 + - 0.1 cm

Measured Values1.703 E-3 + - 6.0 E-5 cm135.8 + - 5.9 cm0.6992 + - 0.0025 cm43.82 + - 0.14 cm3.73 + - 0.13 cm

Calculated Values

0.5 1.47 + - 3.6%1.0 2.92 + - 3.7%1.5 4.38 + - 3.6%

G mt (g)

Heat Pipe Fill Station

• No horizontal lines

• Short runs of large diameter tubing

• Detect and remove trapped vapor by cycling valves

• Fully calibrated

0.5 3.6% 5.0%1.0 3.7% 2.9%1.5 3.6% 1.9%

G Δmt (g) Δmd (g)

Experimental Setup

• 8 ft dia Centrifuge Table

• 20 HP DC motor

• Separate instrumentation and power slip rings

• On-board TC signal conditioning

• Double-pass hydraulic rotary coupling

• Copper-ethanol heat pipe bent to outer radius of centrifuge table

Experimental Setup

Thermocouple placement:

• Unheated/uncooled sections for thermal resistance

• Circumferential and axial distributions in evaporator section for evaporative heat transfer coefficient

Experimental Results

Temperature distributions:

• Uniform temps for low input power levels

• Evaporator temps increase with input power: Partial dryout of evaporator

Inboard

Experimental Results

Thermal resistance vs transported heat:

• For G = 0.5, partial dryout even for low power, Rth decreased with ar

• For G = 1.0 and 1.5, Rth decreased and then increased when dryout commenced

• For G = 1.5, dryout was not reached for ar > 2.0-g

G = 0.5

G = 1.0

Qt (W)

G = 1.5

Experimental Results

Evaporator temperature vs transported heat for ar = 0.01-g:

• Temperature increased with Qt

• For G = 1.0, grooves were full near adiabatic section, dry near evaporator end cap

• Temps converge to the same value around the circumference during dryout

Qt (W)

x = 54 mm

x = 92 mm

x = 130 mm

x = 168 mm

Experimental Results

Evaporator temperature vs transported heat for ar = 10.0-g:

• Dryout was delayed due to improved pumping of helical grooves

• Temperature variation around circumference was greater than ar = 0.01-g

Qt (W)

x = 54 mm

x = 92 mm

x = 130 mm

x = 168 mm

Qt (W)

Experimental Results

Evaporative heat transfer coefficient vs transported heat for ar = 0.01-g:

• he was very low for G = 0.5 due to dryout

• he increased and then decreased as dryout was approached

• For G = 1.0, partial dryout along the axis occurred (he converged around circumference)

Qt (W)

x = 54 mm

x = 92 mm

x = 130 mm

x = 168 mm

Qt (W)

Experimental Results

Evaporative heat transfer coefficient vs transported heat for ar = 10.0-g:

• he was more uniform around the circumference and along the axial direction for G = 1.0

• he was more constant with respect to Qt compared with ar = 0.01-g

Qt (W)

x = 54 mm

x = 92 mm

x = 130 mm

x = 168 mm

Qt (W)

Comparison of Analytical Capillary Limit Model

and Experimental Data

Maximum heat transport vs radial acceleration:

• Qcap increased significantly with ar

• For G = 0.5, heat pipe operated only for ar 8.0-g

• For G = 1.5, capillary limit could not be reached for ar 4.0-g

• Analytical model agrees well with data for G = 1.0

– Assumed full grooves, no liquid communicationar (g)

ar (g)

G = 0.5

G = 1.0

G = 1.5

Conclusions

• Capillary limit increased, thermal resistance decreased significantly with working fluid inventory

• Evaporative heat transfer coefficient was a strong function of working fluid inventory

• Analytical model prediction was good for G = 1.0, but unsatisfactory for underfilled and overfilled heat pipes

Current Research• Thomas, S., Lykins, R., and Yerkes, K., 2000, "Fully-Developed Laminar Flow in Trapezoidal Grooves with Shear

Stress at the Liquid-Vapor Interface," submitted to the International Journal of Heat and Mass Transfer.• Thomas, S., Lykins, R., and Yerkes, K., 2000, "Fully-Developed Laminar Flow in Sinusoidal Grooves," submitted

to the ASME Journal of Fluids Engineering.

• Use results of numerical model to improve analytical capillary limit model for revolving helically-grooved heat pipes

• Numerical model accounts for countercurrent liquid-vapor shear stress and working fluid inventory