Influence of Heat Exchanger Tube Layout on Performance of Heat Pump System for Electric Cars

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 105 (2017) 5085 – 5090

The 8th International Conference on Applied Energy – ICAE2016

Influence of Heat Exchanger Tube Layout on Performance of Heat Pump System for Electric Cars Xiaoqiang Zhanga,b,c, Qingfeng Xued, Huiming Zoub,c*, Jixuan Liub,c, Changqing Tianb,c, Xuelai Zhanga a Shanghai Maritime University, Shanghai 201306, China Beijing Key Laboratory of Thermal Science and Technology and Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, CAS, Beijing 100190,China c Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China d China FAW Group Corporation R&D Center, Changchun 130011, China b

Abstract

The external micro-channel heat exchanger plays a significant role in performance of the heat pump system for EVs. It functions as both the evaporator and condenser. A test bench with the heat pump system is set up to research the influence of arrangement (vertical or horizontal) of flat tubes on the system performance. The investigation results show: on heating conditions, the horizontal HX has greater pressure drop, which brings about less heat absorption, higher compressor power consumption and worse system COP. However, the vertical HX is easier to frost than the horizontal. On cooling conditions, the heat pump system with horizontal external HX has larger cooling capacity and better COP. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.

Keywords: Electric Vehicle, microchannel heat exchanger, vertical, horizontal

1. Introduction Heat pump system is a potential development direction of the automobile air conditioning because of its characteristic of high efficiency and energy saving. External micro-channel heat exchanger is a crucial component which needs both bearing the function of condensing and evaporating. Its performance which is easily affected by ambient environment has a strong effect on the heat pump system performance. Thus, it`s been a researching focus. Yan et al. [1] experimentally studied the influence of uneven distribution of temperature on the microchannel heat exchanger used as the condenser and evaporator. The results show: the influence of uneven

* Corresponding author. Tel.: +86-10-82543697. E-mail address: [email protected].

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.1030

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distribution of temperature on the heat pump performance accounts for 3.5% and 7.3% respectively and numbers of passes have different effects on the condenser and evaporator. Xu et al. [2] studied influences of fin parameters on performance of the heat exchanger. The results show: the spacing of fins has a strong impact on heat transfer and reducing the spacing of fins can increase heat transfer amount by 17%. Increasing the height of fins doesn`t have obvious influences on performance of the heat exchanger, but it can save 20% of costs. Yuan et al. [3] used the method of uniform design to build a model, chose length, height of the microchannel as considering factors and analyzed influences of these factors on uniform distribution of flow in the micro-channel. Nae-Hyun et al. [4] experimentally studied the effect of three different inlet configurations (parallel, normal, vertical) on the refrigerant distribution in a parallel flow mini-channel heat exchanger. The investigation results show: The flow distribution is better for normal or vertical inlet configuration than for the parallel inlet configuration. Normal or vertical inlet yields approximately similar flow distribution, although slightly better results were obtained for normal inlet at high mass fluxes or high qualities. However, when it comes to effects of arrangement of flat tubes on heat exchangers performance, there is few literatures and researches. In this paper, An experimental bench is set up to study the influence of flat pipes layout of the microchannel HX(horizontal/ vertical) on the performance of heat pump air-condition system. As well, the heat transfer capacity and pressure drop of these two types of HX are calculated by simulation based on Dymola, hoping to provide a reference on structural design of microchannel heat exchangers. Nomenclature V

Vertical

H

Horizontal

L

Length of flat tubes

Dh

Hydraulic diameter

μl

Dynamic viscosity of the liquid phase

f

Friction coefficient

h1

Heat transfer coefficient for evaporating

h2

Heat transfer coefficient for condensing

ΔP

Experimental result for pressure drop

ΔP’ Simulation result for pressure drop

Q

Experimental result for heat transfer

Q’

Simulation result for heat transfer

2. Experimental bench setup Fig.1 shows the diagram of the experimental bench system, including several main components, test samples and measurement devices. The structure and parameters of test samples are shown in fig. 2 and table 2.The measurement devices consist of sensors, which is used to measure temperature, pressure and mass flow rate of the refrigerant. Table 1 shows the basic parameters of the measurement devices.

Vertical

Fig. 1.Schematic diagram of the heat pump system of test bench

Horizontal

Fig. 2. Diagram of test sample structure

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Table 1. Measurement devices

Table 2. Parameter of test samples

Parameter

Type

Range

Error

Types

Vertical

Horizontal

Temperature

Thermocouple

-40 to 125ć

±0.5ć

Size/mm

580x430x16

580x430x16

Pressure

Diaphragm

0 to 30bar

±0.5%

Distribution of flow

Air speed

Hot bulb

0 to 20m/s

±3%

channel

Single

Double(1:3)

Mass flow rate of Ref

Coriolis

<200kg/h

±0.1%

Number of flat tubes

72

13-39

3. Simulation A model of external micro-channel HX for EVs is established through Dymola and then its pressure drop and heat transfer under both heating and cooling conditions are calculated. Main equations applied to calculate are shown in table 3. Table 3. Equations of mathematical model Heat transfer equation

I

Heat balance equation

I qma c pa tao  tain qmr c pr  tro  trin

Heat transfer coefficient for evaporating

h1

Pressure drop for evaporating

'P1

Heat transfer coefficient for condensing

h2

Pressure drop for condensing

'P2

KA'tm

30Re0.857 Bo0.714 1  x

4f

L Re2 fl Dh3 2 Ul

0.0265

4f

Ol Dh

0.143

Ol / Dh

, f 0.435Re0.12 fl

Re0.8 Prl1/3

L Re2 Pl2 Dh3 2 Ul

4. Results and discussions 4.1. Heating condition Figure 3 displays the performance comparison of the heat pump system with respectively the vertical and horizontal outdoor HX from the experimental and simulation results under heating conditions. Ref. mass flow rate is almost the same, however, horizontal HX having a greater pressure drop on ref. side, as shown in fig. 3(a) and (b). This mainly results from structural difference: with the identical windward area and ref. mass flow rate, the horizontal HX has longer and fewer flow paths (fig. 2), which results in higher velocity of flow and larger flow resistance. Because of the lower pressure drop of the vertical HX, it has larger enthalpy, heat transfer and lower compressor power consumption than horizontal HX, as shown in fig. 3(c), (d) and (e). And experimental and simulation results for pressure drop and heat transfer are generally consistent. Errors are almost within 10%, as shown in fig. 3(c) and (d). Based on all the above factors, a better COP of the heat pump system under heating conditions is obtained for vertical HX, as fig. 3(f) shows. The COP averagely increases by about 12%. Fig. 4 displays frosting conditions of vertical and horizontal HX after 2 hours at -5 ć, 60% relative humidity outdoor environment. For the vertical HX, thick frost nearly covers the whole area except the small part around the outlet, where ref. has become superheated state. For the horizontal HX, some thin

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frost just emerges on the bottom. This phenomenon may be related to the flow path arrangement of horizontal HX as fig. 2 shows. From inlet to outlet, the distribution of flow path numbers is 1:3, and ref.

(a)

(c)

Ref. mass flow rate

Pressure drop on the ref. side

(e) Compressor power consumption

(b)

Δh between inlet and outlet of the HX

(d)

Heat transfer

(f) COP

Fig. 3. Performance comparison between vertical and horizontal external HX

Xiaoqiang Zhang et al. / Energy Procedia 105 (2017) 5085 – 5090

in the most flat tubes on the upper part of the HX is in superheated state. Thus, there is nearly no frost occuring on the top of the horizontal HX. At the same ambient temperature and relative humidity below 30%, no frosting occurs for both types of HXs.

(a) Vertical HX

(b) Horizontal HX Fig. 4. Frosting conditions of both HXs

4.2. Cooling condition Under cooling conditions, the heat pump system with horizontal external HX has better cooling capacity and COP. The COP averagely increases by 30%, much larger than that on heating conditions. On cooling conditions, for the vertical HX with inlet at the bottom and outlet at the top, coagulating liquid flows downward under the influence of gravity and impedes the ref. vapor flowing upward, which leads to heavier pressure drop, lower heat transfer and higher compressor power consumption and then along with lower cooling capacity and COP, as fig. 5shows. In addition, fig. 5(a) and (b) show: experimental and simulation results for pressure drop and heat transfer are very close under cooling conditions. All errors are within 10%.

(a) Heat transfer

(b) Pressure drop on the ref. side

(c) Cooling capacity and power consumption

(d) COP

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Fig. 5.Performance comparison between vertical and horizontal outdoor HX

5. Conclusion According to the above experimental research, influences of arrangement of flat tubes on the performance of the heat pump system are investigated. Listed below are major conclusions: (1) On heating conditions, the vertical HX is a preferable choice for the outdoor HX when it comes to heat transfer performance. The COP is about 12% higher than that of horizontal HX. This is because the horizontal HX has longer flow paths and then higher pressure drop. However, considering the factor of frosting, the horizontal HX has better performance. (2) On cooling conditions, horizontal external HX brings lower compressor power consumption, greater cooling capacity and higher COP. The COP increases by about 30%, larger than that on heating conditions. This may result from the reason that under cooling conditions, the coagulating liquid flowing downward and the ref. vapor flowing upward in the vertical HX tubes resist with each other and then along with higher pressure drop. (3) Considering two types of HXs have their own advantages under cooling and heating conditions, choices should be made according to the above findings, the ratio of cooling and heating load, frosting and other factors in practical applications. Acknowledgements We would like to thank the support by the Natural Science Foundation of China (No. 51676201) and External Cooperation Program of BIC, Chinese Academy of Sciences (No. 1A1111KYSB20130032). References [1] Yan R D, Xu B, Chen J P, et al. The Impact on Air Conditioning System of Two-phase Distributionin Microchannel Heat Exchanger. Journal of Refrigeration, 2013, 34(3): 20-24. [2] Xu B, Qi Z G, Chen J P, et al. Simulation and Parametric Analysis of Microchannel Condenser . Refrigeration Technology, 2011, 31(4): 16-20. [3] Yuan P, Sun B, Lv Y L, et al. The Sensitivity Analysis of fluid Distribution Uniformity on the Structure Parameters for Microchannel Heat Exchanger. Journal of Light Industry, 2016, 35(3): 74-80. [4] N. H. Kim, D. Y. Kim, H. W. Byun. Effect of Inlet Configuration on the Refrigerant Distribution in a Parallel Flow Minichannel Heat Exchanger. International Journal of Refrigeration, 2011,34: 1209-1221.

Huiming Zou Got doctor degree of refrigeration and cryogenic engineering in University of Chinese Academy of Sciences in 2010, worked as an associate professor in Technical Institute of Physics and Chemistry, Chinese Academy of Science since 2010. Research fields: automotive air conditioning, linear compressor, heat pump