numerical investigation into thermal...

Ahmet Serhan Canbolat, Burak Türkan, Akın Burak Etemoğlu, and Muhiddin Can, JMAS Vol 4 Issue 1 2016

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MACROJOURNALS

The Journal of MacroTrends in Applied Science

NUMERICAL INVESTIGATION INTO THERMAL COMFORT CONDITIONS IN A MIDIBUS Ahmet Serhan Canbolat, Burak Türkan, Akın Burak Etemoğlu, and Muhiddin Can Uludag University, Engineering Faculty, Mechanical Engineering Department. BURSA

Abstract In this study, midibus, which are often used local and intercity public transportation is modelled. Thermal comfort, velocity and temperature distribution are investigated numerically. Calculations are made by a software package, which is used the finite volume method, under three-dimensional and steady state conditions. A vehicle cabinet in actual size is used in the analysis. The passenger and the driver are separately modelled and thermal condition in their surrounding evaluated in terms of thermal comfort. Percent dissatisfaction value in the study is described as the thermal comfort parameter. Percent dissatisfaction values for different rate of injections and temperature are calculated separately and tried to determine the appropriate values. Standard k-ε, as the turbulence model and Standard Wall Function as the wall approach are used in the analysis.

Keywords: Computational Fluid Dynamics, Thermal comfort, Temperature distribution in cabinet

1. Introduction

One of the most important effect of technologic development on human being is comfort. The expected criteria of the public transportation vehicles are changing with developing technology continuously. Ventilation inside the cabinet is one of these parameters. Most of the people of socially and economically developed countries spend 90% of the day in air conditioning indoors. [1]. This situation is ever increasing the studies related to thermal comfort. Thermal comfort as a definition is an objective assessment that allows people to express the satisfaction and evaluation about thermal environment [2].


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It should not be forgotten while investigating thermal comfort, this term is a concept related to feelings and emotions and depends on ambient temperature, relative humidity, air movement in the environment, radiation temperature, one’s mobility and the clothing factor [3].

In the study of Zhu and his colleague[8] made they numerically investigate air movement inside a bus cabinet for different ingress and egress locations and in the study of John and his colleague[9] made, they modelled a bus and they numerically investigate in what way different window sizes effect on the air velocity inside the bus.

In this study, a solution field on the basis of midi-geometry, which is commonly used in public transport in the city, is formed. Different inlet temperature and velocity status of the thermal comfort and ventilation inside the cabinet are investigated numerically.

2. Modelling and Solution Method

2.1. Geometric Modelling

In the study, a midibus-type vehicle geometry are taken as a basis. To ventilation mode and vents, different methods and performances are tried in the literature. The air vents are located on the passenger seats in small holes. The outlet hole at the rear of the cabinet is modelled circularly. A total of ten people except driver are modelled. Due to the fact that the cost of the hardware and networking through people’s body form is high, cubic modelling is made. It gives similar results and tested in literature. [4]

Figure 1. Solid model of the midibus carrying people

Midibus is modelled as 1.76 m in height, 7.73 m in length and 2.215 m in width. It is thought that the diameter of the vents through which the air blown as 5 cm, the diameter of the outlet hole as 50 cm. the studied simulations obtained by ANSYS FLUENT 14.5 software, which is running with the finite volume method.


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2.2. Mesh Structure

Mesh structure is formed by tetrahedral element structure. (Figure 2)

Figure 2. Divided state of the flow field of the finite elements

The network areas in which network with 246000 elements number have much more common structure. (Figure 3)

Figure 3. Detailed meshes into the air inlet section

2.3. CFD Approach

In this study air flow in the cabinet three dimensional Reynold- averaged navier –stokes (RANS Momentum), and continuity equations by numerical methods is used. Standard k-ε model is selected as turbulence model. It appears that k-ε model has a wide range of applications in the literature and being used frequently in the room ventilation. The studies showed that these model gives very similar results of experimental studies [5]. The convergence criteria considered in the analysis are shown in the following table.


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Table 1. The convergence criteria adopted for each parameter.

Parameters Convergence Criteria

Continuity 1.10-3

Velocity (x-direction) 1.10-3

Velocity (y-direction) 1.10-3

Velocity (z-direction) 1.10-3

Energy 1.10-6

k 1.10-3

Epsilon (ε) 1.10-3

2.4. Boundary Condition

The study is conducted under the steady state assumptions. In studied scenarios is aimed at cooling under hot environmental condition in the summer. Windows and doors are assumed constant temperature at 30 ºC.

Figure 4. Inlet and outlet holes and demonstration of windows on the model.

There are eleven people in the vehicle. And boundary condition for the people is selected as the constant surface temperature. Human body surface temperature is adopted as 34 ºC. The inlet temperature and velocity values are systematically examined in different values and expressed at Table 2.


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Table 2. Different air blowing velocity and temperature examined in the analysis.

Temperature (oC) Velocity (m/s)

18

1

1.5

2

2.5

3

22

1

1.5

2

2.5

3

26

1

1.5

2

2.5

3

3. Calculations and Discussion

3.1. Percent Dissatisfaction

If the temperature of the environment of people location is above the desired comfort, people are able to perceive the draft as a pleasant breeze. But the people who feel cold in the same conditions, are able to find the draft very disturbing.

Draft defined as percent dissatisfaction in the literature or draft rate and in equation 1 shown as a mathematical formulation. . [6] One of the article published by Fanger in 1989. [7], Draft referred as follows;

PD = (34-T)(V-0.05)0.62(0.37V Tu + 3.14) (1)

PD = Percent Dissatisfaction (%)

Tu = Turbulence Intensity(%) (%10-60)

T = Temperature (ºC) (20-260)

V = Velocity (m/s)

The use of equation number 1 is restricted in some cases. More specifically, if the air velocity smaller than 0.05 m/s, this velocity must be synchronized at 0.05 m/s. Moreover the PD values below zero is equalized to zero as well. When the PD is above 100%, the value is stabilized at 100%. According to Ashrae standard, PD value for 15% and below adopted as comfortable.


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Percent dissatisfaction distribution obtained for 26ºC is shown in Figure 6. It is assumed that an increase in velocity causes an increase in PD too. The results obtained in PD distributions, the area, where velocity value is high, the PD values are high as well.

PD values changes, in different velocity and temperature obtained 5 cm above from the passenger, is shown in the graph.

Figure 5. Percent dissatisfaction values obtained at different velocities and temperatures.


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Figure 6. Percent dissatisfaction distributions obtained for different velocity in the midibus (1-1, 5-2-2, 5-3 m/s) respectively.


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3.2. Streamlines and Temperature Distribution

To set an example, results obtained from flow lines for 1-1, 5-2-2, 5-3 m/s at 26ºC are shown below; (Figure 7). Moreover for 3 different temperature (18 -22 -26 ºC) to set an example, temperature distribution in 1 m/s is shown below (Figure 8).


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Figure 7. Streamlines obtained for different velocities in midibus (1-1, 5-2-2, 5-3 m/s) respectively.


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Figure 8. Temperature distribution obtained for different temperature in the midibus (18 -22 -26 ºC) respectively.

4. Conclusions

In this study for three different blowing temperature and five different blowing velocity, the draft values are calculated numerically. Temperature distribution, draft values and streamlines are shown in selected plane. Draft value obtained from any point of head level of passengers and these values are shown at Figure 5. The results obtained from this study are listed as below;

The presence of draft values in the vehicle is important for people to thermal comfort.

As it can be seen from Figure 5 air blowing in 2-2,5-3 m/s, at 18 ºC and air blowing in 3 m/s, at 22 ºC, thermal comfort is not provided for the passengers.

When the air blowing at 26 ºC any velocity value up to 3 m/s does not create any adverse effect on the thermal comfort

If it will be blown at lower temperature, air is to be blown at low temperature to provide thermal comfort.

Moreover the position of inlet and outlet holes could have an effect on airflow and draft values.


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References

1. Hoppe, P., Martinac, I., “Indoor climate and air quality”, Int. Journal of Biometeorol, 42: 1-7, 1998

2. ASHRAE Standard 55, Handbook of Fundementals .

3. Atmaca, Y., Yiğit, A., Isıl Konfor ile ilgili Mevcut Standartlar Ve Konfor Parametrelerinin Çeşitli Modeller İle İncelenmesi. IX. Tesisat Mühendisliği Kongresi, 6-9 Mayıs 2009.

4. Daithankar, N., Udawant, K., and Karanth, N., "Prediction of Thermal Comfort Inside a Midibus Passenger Cabin Using CFD and Its Experimental Validation," SAE Technical Paper 2015-26-0210, 2015, doi:10.4271/2015-26-0210.

5. Yüce B.E., Pulat E.,Oda Havalandırmasında Isıl Konforun Sayısal Simülasyonu, 2010, 12. Ulusal Tesisat Mühendisliği Kongresi, İZMİR

6. Fanger, P.O., A.K. Melikov, H. Hanzawa, and J. Ring. 1989. Turbulance and draft. ASHRAE Journal 31(4):18-25

7. Zhu, S., Srebric., J., Spengler., J., D., Demokritou., P., “An advanced numerical model for the assessment of airborne transmission of influenza in bus microenvironments.” Building and Environment 47 (2012)

8. John, P., Sriram, B., Kumar R.S., Kumar, S.V. Ramasamy. P., Ram. C.V., “Ventilation Improvement in a Non-AC Bus.” SAE International, 2013.

9. M, Simion., L, Socaciu., P, Unguresan., “Factors which Influence the Thermal Comfort Inside of Vehicles” Energy Procedia Volume 85, January 2016, Pages 472–480

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numerical investigation into thermal...

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