Heat transfer characteristics of the integrated heating system for cabin and battery of an electric vehicle under cold weather conditions

International Journal of Heat and Mass Transfer 117 (2018) 80–94

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Heat transfer characteristics of the integrated heating system for cabin and battery of an electric vehicle under cold weather conditions Jae-Hyeong Seo a, Mahesh Suresh Patil a, Chong-Pyo Cho b, Moo-Yeon Lee a,⇑ a b

School of Mechanical Engineering, Dong-A University, 37 Nakdong-Daero 550, Saha-gu, Busan, Republic of Korea Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea

a r t i c l e

i n f o

Article history: Received 8 July 2017 Received in revised form 2 October 2017 Accepted 2 October 2017

Keywords: Battery thermal management Cabin air heater Electric vehicle Heat transfer Heating system

a b s t r a c t The objective of this study is numerically to investigate the heat transfer characteristics of the integrated heating system considering the temperature of cabin and battery of an electric vehicle under the cold weather conditions. The integrated heating system consists of a burner to combust fuel, an integrated heat exchanger for CHE (coolant heat exchanger) and AHE (air heat exchanger). The heat transfer characteristics like the overall heat exchanger effectiveness, the heat transfer rate, the temperature distribution and the fluid flow characteristics like the pressure drop, velocity distribution of the investigated integrated heating system were considered and analyzed by varying the inlet mass flow rates and the inlet temperatures of the cold air and water, respectively. The average Nusselt numbers for the cold air side and the water side were increased 28.4% and 9.5%, respectively, with the increase of the cold air side Reynolds numbers from 15,677 to 72,664 and the water side Reynolds numbers from 4330 to 11,912. The numerical results showed good agreement within ±9.0% of the existed data and thus confirmed that the present model was valid. In addition, the proposed integrated heating system could be used as the thermal management of the cabin and the battery system of the electric vehicle under the cold weather conditions. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Environmental degradation caused by excessive use of fossil fuels has resulted in the sharp growth of worldwide interest in the development of the eco-friendly transportation. The transportation field is one of the high-consuming industry of the primary energy, as it represents 30–35% of the total primary energy needs for most of the industrialized countries [1]. In 2007, the dependency on oil as fuel was observed to be as high as 95% for private transport, and it accounted for almost 50% of total oil consumption [2]. Therefore, it would obviously become increasingly difficult for the transportation industry to comply with stringent emission standards for solving an international environmental issue. These changes in the industry have resulted in shift of interest of the automotive companies to the green vehicles, such as hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV) and fully pure electric vehicle (EV), as a promising option for transport in the future. Due to the continuous revision of international rules and regulations for fossil fuel usage and environmental concerns to reduce the carbon footprint, many ⇑ Corresponding author. E-mail address: [email protected] (M.-Y. Lee). https://doi.org/10.1016/j.ijheatmasstransfer.2017.10.007 0017-9310/Ó 2017 Elsevier Ltd. All rights reserved.

automotive industries have developed battery-operated electric vehicles to assist or replace ICE. As a result, much research on EVs (electric vehicles) been conducted in the automotive industry and related academia to develop high-performance EVs. Though internal combustion engine (ICE) operated vehicles are the most popular, they have an inferior efficiency of only around 40% [3]. Despite the higher efficiency, no emission of the polluting substances and a near zero carbon footprint of EVs driven by batteries, EVs are still struggling to solve the issues such as safety, vehicle range, cost and reduced battery performances at extreme temperatures [4,5]. Especially, the vehicle range of EVs is the most important reason for the slow market penetration of EVs. Although the vehicle range can be increased by increasing the battery capacity of EVs, the weight, volume and cost of the vehicle could be increased. EVs have the limited availability of charging infrastructure in comparison with ICE vehicles, which have a long vehicle range with easily available fuel refill systems [6]. Moreover, airconditioning (AC) system for cooling and heating the EVs cabin to improve the passenger’s thermal comfort consume a considerable amount of battery power of EVs, thereby, sharply reducing the vehicle range of EVs. A significant amount of research on increasing the vehicle range in cold and hot conditions has focused

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Nomenclature AC AHE ASHP CAD CHE COP EHR MW PTC P Q_ _ m D T

q T U F D d h SM SE Dp

air conditioning air heat exchanger air source heat pump computer aided drawing coolant heat exchanger coefficient of performance exhaust heat recovery molecular weight positive thermal coefficient static (thermodynamic) pressure (Pa) heat transfer rate (W) mass flow rate change in quantity temperature (°C, K) density (kg/m3) static (thermodynamic) temperature average velocity (m/s) weight function for blending characteristic length identity matrix or Kronecker delta function enthalpy momentum source energy source pressure drop (Pa)

on reducing the power used for cooling and heating the cabin of EVs. Lee et al. [7] conducted experimental studies to investigate the effect of full-load AC on driving characteristics and found that the driving range was reduced by up to 16.7% and 50% for cooling and heating, respectively. For ICE operated vehicles in general, the exhaust heat is collected by an EHR (exhaust heat recovery) system since burning fuel produces a quite large amount of exhaust heat. Hatami et al. [8] conducted a numerical study of the EHR of ICE operated vehicles and reported that the heat is recovered in the range from 0.6 kW to 5.9 kW, depending on the engine speed. This value could be as large as 18 kW for high-capacity vehicles (around 90hp) like the 13B Toyato [9]. Currently, PTC (positive thermal coefficient) heater is widely used as auxiliary heating in ICE vehicle and EVs. The important issue with PTC heating technology is high cost with high power (>2 kW) along with higher energy consumption. It is reported that the PTC heater could lead up to 24% of loss in driving range of fully charged EVs [10]. Ayartürk et al. [11] observed that, depending on COP, 5.5 kW of electric power is consumed to heat a cabin space when a PTC system is used. Assuming an EV battery capacity of 28 kWh and average power consumption of 20 kW while cruising at a constant velocity of 100 km/h, an EV could have a maximum range of 140 km. If an additional 5 kW of power is consumed to heat the cabin, then the vehicle range decreases to 112 km, a sharp decrease in range of 20%. This indicates that efficiently heating the cabin space of EVs could have a substantial influence in preventing a decrease in the vehicle driving range. ASHP (air source heat pump) is another option for cabin heating in EVs. An electric heater system based on PTC could be installed at a low cost, but under operation draws a significant amount of battery power since it is electric conversion-based system, resulting in a dramatic reduction in vehicle range. Many researchers have also investigated the performance of a heat pump system for vehicular thermal management [12,13]. In case of PTC heater, the ratio of heat output to electric input is less than 1.0, whereas, coefficient of performance (COP) for a heat pump is larger than 1.0, indicating a heat pump system as a reasonable method to

s l lt hc k k

x r e Cp

ai bi Nu x, y, z


r

stress tensor dynamic viscosity turbulent viscosity average heat transfer coefficient W/(m2 K) thermal conductivity (W/mK) Turbulence kinetic energy per unit mass angular velocity Prandtl number effectiveness specific heat capacity (J/(kg K)) coefficient in BSL RS model coefficient in BSL RS model Nusselt number Cartesian coordinates dyadic operator gradient operator

Subscripts in inlet out outlet h hot gas c cold air w water

enhance the thermal comfort in EVs. In EVs, when the PTC heating element is employed for heating cabin air, all power is drawn from the battery power source. Many early developed EVs like Mitsubishi i-MiEV [14,15] and Nissan LEAF [16] used this type of heating system. The heat pump system provides sufficient amount of heat to an incoming air stream under mild weather conditions, nevertheless its heating capacity drops under more severe conditions when the outside temperature is lower [17]. Qi reviewed state-of-the-art advances in air conditioning and heat pump system in electric vehicles. Although, heat pump systems are efficient, there are few challenges in air conditioning heat pump systems [18]. A heat pump uses atmospheric heat, which makes it highly efficient as they provide more thermal energy than the input energy [19]. There are many studies related to the performance and feasibility of heat pump system in electric vehicles. Kim et al. found that although CO2 heat pump increased heating capacity by 35–54% and COP increased by 16–22% for evaporator front arrangement, cooling capacity decreased by 40–60% and COP decreased by 43–65% for the same arrangement [20]. Kim et al. suggested that heat pump system with heater core could enhance the performance of the heating system [21]. Cho et al. explored the heating performance of the coolant source heat pump for an electric bus, which could use exhaust heat from its electric devices [22]. Lee et al. investigated performance of stack-coolant source heat pump using R744 for fuel cell electric vehicles and found that heating capacity of 5.0 kW was attained [23]. Lee et al. investigated the performance of mobile heat pump for the large passenger electric vehicle and found that cooling and heating is sufficient under ambient temperatures [24]. The above studies have been conducted at ambient or lower temperatures, which points out that the heat pump is effective for cooling as well as heating. However, many researchers found that although a heat pump heater system is more efficient than PTC heating element, heat pump performance reduced severely during extreme cold condition [17,25,26,27]. Relatively lower COP under very cold conditions with a very thick frost layer formation on the exterior heat

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exchangers when ambient temperature decreased drastically was a major reason [12]. In addition, the common structure of 4-way valve consists of a moving block inside and considering the severe vehicle operation conditions under long-term vibration, the block might not function properly. In addition, the performance deterioration of the batteries operated under extreme thermal conditions has been reported. In countries with a cold climate, the degradation of battery performance at low-temperature operation of EVs hampers their widespread acceptance and competition with ICEs. Recently, lithium-ion batteries are commonly used as a power source for EVs. Even though a lithium-ion battery has a high energy-to-weight ratio (180 Wh/ kg) and a high power-to-weight ratio (1500 W/kg) with a low self-discharge rate [28,29], the performance of a lithium-ion battery decreases at low operating temperatures due to the slowdown in their chemical reactions [30–32]. Subzero temperature condition changes power capacity and performance of Li-ion battery which is more common as power source in EVs in current times [33,34]. Jaguemont et al. conducted a comprehensive review of subzero temperature performance of Li-ion battery used in EVs and compared various methods such as air heating, liquid heating and phase change materials as heating strategy [34]. To address this issue of reduced performance of Li-ion batteries at low temperature, some researchers have suggested warming the battery cell stack during operation in cold weather by using an external heat source or the battery itself. However, if the battery power of an electric vehicle is used for warming the battery cell stack, it could have an adverse effect on the vehicle driving range [4]. The existing studies for cabin heating of an electric vehicle at low ambient temperature have shown that both PTC heating method and heat pump system were ineffective. The PTC method consumed more battery power of an electric vehicle and heat pump system performance degraded sharply at low ambient temperatures [35]. In addition, a separate system disconnected from the electrical power system of the battery was needed for an efficient battery thermal management and ensuring the better performance of EVs. As identified in the above discussion, the issues of cabin heating and battery thermal management in EVs must be resolved to have competitive edge with traditional vehicles. Correspondingly studies on innovative technologies other than existing technologies such as PTC method and heat pump system must be undertaken. In order to resolve the battery-power consumption problem of the heating system at lower ambient temperatures, the integrated heating system for the cabin and battery of an electric vehicle could be used for cabin heating and battery heating. However, there are very few studies on the heat transfer character-

istics of the integrated heating system for a cabin and battery for an electric vehicle. Therefore, the objective of this study is numerically to investigate the heat transfer characteristics of the integrated heating system for cabin and battery for an electric vehicle under the cold weather conditions. The heat transfer characteristics like overall heat exchanger effectiveness, heat transfer rate, temperature distribution and fluid flow characteristics like pressure drop, velocity distribution of the investigated integrated heating system for the cabin and battery were considered and analyzed by varying inlet mass flow rates and inlet temperatures of the cold air and water, respectively. In addition, the investigated integrated heating system for the cabin and battery of an electric vehicle could be considered as a promising option for cabin heating and battery thermal management.

2. Numerical analysis The numerical model used in this study included certain physical assumptions, meshing and grid independence, boundary conditions, governing equations and turbulence models. The grid independence showed that the results were insensitive to the grid numbers. The particular mesh is selected according to the computational cost and grid independency. The boundary conditions are properly considered based on the previous studies and the particular turbulent models are used and justified considering the flow parameters and geometrical limitations. 2.1. Physical model Fig. 1 shows a schematic of the integrated heating system for cabin and battery of an electric vehicle. The suggested model was developed by integrating the AHE (air heat exchanger) and the CHE (coolant heat exchanger) to maintain the temperature of the cabin air and the battery, respectively. In addition, the numerical domain for the analysis consisted of the entire heat exchanger part as highlighted in the Fig. 1. The integrated heating system had the overall size of 495 mm  120 mm  120 mm and 21 number of fins in the AHE (air heat exchanger) with the 5.0 mm fin pitch and the 2.0 mm fin thickness and the CHE (coolant heat exchanger) with the 2.5 mm fin pitch and the 0.5 mm fin thickness. For heating the water, a tube of 8 mm internal diameter with 1 mm thickness was provided. The specifications of the integrated heat exchanger used in the developed heating system for cabin and battery showed in Table 1.

Fig. 1. Schematic diagram of the integrated heating system for cabin and battery of an electric vehicle.

J.-H. Seo et al. / International Journal of Heat and Mass Transfer 117 (2018) 80–94 Table 1 Specifications of the integrated heating system. Specifications

Values

Total size (mm) Top side number of fin Top side fin pitch (mm) Top side fin thickness (mm) Tube inside diameter (mm) Tube outside diameter (mm) Bottom side number of fins Bottom side fin pitch (mm) Bottom side fin thickness (mm) Top side number of baffle

495  120  120 21 5 2 8 9 21 2.5 0.5 6

Table 2 Specifications of the butane fuel. Specifications

Values

C atoms H atoms MW (kg/kmol) Fuel density (kg/L) (liquid state)

4 10 58.1234 598.4

As shown in Fig. 1, the outlet part of the burner for supplying the hot gas to the integrated heat exchanger was located in the considered numerical domain. The hot gas from the burner during combustion process showed the outlet average temperature range of 900–1000 °C and composed of CO2, H2O, N2 and O2. Table 2 showed the specifications of the butane fuel [36,37]. The fuel was combusted in a burner with 10% excess air and the excess air was provided to have complete combustion to optimize the fuel usage. The cold air need to be heated to supply the warm air to the vehicle cabin, and it was first flown over the burner using a fan. Then, to increase the temperature of the cold air more, the same air entered the AHE of the integrated heat exchanger. The water was simultaneously flown in tube to run through the CHE part of the integrated heat exchanger. The heated water then could be flown around the battery pack surface to maintain the temperature of the integrated heating system under the cold weather conditions. The inlet mass flow rate and inlet temperature conditions were provided for the inlet cold air, the inlet hot gas, and the inlet water. All inlet conditions were provided with the mass flow rate as the input boundary conditions. All outlets were set to pressure conditions of atmospheric pressure. The inlet cold air flow was assumed to be incompressible, and the thermophysical properties for the same were considered to be constant, especially, thermal conductivity and viscosity. The inlet cold air, the hot gas, and the water temperature continuously changed according to the position in the domain, as suggested by Bang et al. [38] and Seo et al. [39]. The turbulent intensities were set to 5% for all the cases. This was selected owing to the geometry and relatively rapid hot gas movement with various changes in direction creating a low-medium level turbulence. For the outer wall surface, adiabatic boundary condition was used. The computer aided design software SolidWorks 2015 was used to create integrated heat exchanger model. ANSYS 16.2 was used to mesh the model and solve the equations Table 3 Specifications of the mesh. Mesh description Number of nodes Number of elements Tetradedra Wedges Pyramids Hexahedra Polyhedra

Percentage (%) 1,797,138 6,159,298 5,627,970 167,432 47,936 315,960 0

– 100 91.37 2.71 0.77 5.13 0

83

for the turbulent fluid flow with the conjugated heat transfer. ANSYS CFX is a fluid dynamics program that solves the fluid flow and heat transfer equations based on the finite volume method [40]. Different meshing schemes were used to improve the accuracy of the simulation. Due to the complexity of the structures of the investigated model involving curved faces and closely situated fins and other structures, unstructured meshing was used including different types of control volumes, such as tetrahedrons, hexagons, wedges, and pyramids. However, the maximum percentage of the grids were made up of tetrahedrons, as shown in Table 3. The geometry and meshing of the integrated heating system for cabin and battery of an electric vehicle is shown in Fig. 2c and d, respectively. The X, Y and Z co-ordinate directions are indicated in Fig. 2c. The grids around the curved surfaces and near edges were refined to capture accurately the heat transfer and fluid flow phenomenon. In addition, a higher numbers of fine grids (inflation layers) were provided to capture the viscosity effect and boundary layer phenomenon near the tube surface for the water. After meshing 9,824,203 elements were generated with 2,403,917 nodes. Although the two heat exchangers initially seem to be symmetrical, they have different arrangements in the fins in terms of length, width, gap, and numbers. Some mesh refinements along the inner boundaries and surfaces of the model were conducted to meet the requirements of wall functions [41]. The enhanced wall function, which combines two-layer approach in which linear blending function is used for laminar and for turbulent logarithmic blending functions are used [42]. In addition, advanced proximity and curvature functions were used to refine the mesh. The final refined mesh was adopted by comparing the mesh independence test and the computational time. Fig. 2a and b showed the investigated heat exchanger model used in the integrated heating system. To assure the accuracy and validity of the numerical results, an analysis was carried out for the mesh independence of the numerical solution. The mesh dependence was performed with considering the average temperature and pressure drop. Table 4 showed the mesh types with the number of the elements and nodes for the mesh independence. Different types of the mesh were considered with varying element numbers and nodes, as shown in Table 4. Fig. 3 showed as the element numbers increased from 6,159,298 to 14,879,838 the average outlet temperatures at air and water and pressure drop were converged within 1% after the element number of 9,824,203. As a result, the mesh type 3 with the element number of 9,824,203 and the node number of 2,403,917 was selected for numerical analysis considering the accuracy and computational cost. 2.2. Governing equations The investigated model consisted of the cold air, the hot gas, the water and the heat exchanger. The fluid flow assumed three dimensional, incompressible, steady state and turbulent. The hot gas flow was assumed turbulent due to the hot gas movement out of the burner after the combustion. The combustion process created the small explosion and the product of hot gas moved in the complex geometry as shown in the Fig. 1. The cold air and water were considered turbulent. This model was based on the numerical solution of the continuity Eq. (1), the momentum Eq. (2) and the energy Eq. (3) [42–44]. Continuity equation:

@q þ r  ðqUÞ ¼ 0 @t

ð1Þ

Momentum equation:

@ðqUÞ þ r  ðqU
UÞ ¼ rp þ r  s þ SM @t

ð2Þ

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Fig. 2. Integrated heat exchanger used in the developed heating system for cabin and battery (a) Integrated heat exchanger model, (b) Cross section of the integrated heat exchanger model, (c) Simulation domain and (d) Meshing.

Table 4 Mesh types for the grid independence test. Mesh type

Number of elements

Number of nodes

Mesh Mesh Mesh Mesh

6,159,298 7,467,908 9,824,203 14,879,838

1,797,138 1,876,846 2,403,917 3,255,276

1 2 3 4

where the stress tensor, s, is related to strain rate by




2 3



s ¼ l rU þ ðrUÞT  dr  U Energy equation:

@ðqhÞ þ r  ðqUhÞ ¼ r  ðkrTÞ þ s : rU þ SE @t

ð3Þ

where s : rU is a viscous dissipation term. The heat transfer rate at the outlet exhaust gas, outlet cold air and outlet water in the integrated heating system for cabin and battery of an electric vehicle were calculated using Eq. (4), considering the average specific heat at constant pressure, the area average temperatures and area average mass flow rates.

_ p ðT out  T in Þ Q_ ¼ mC

ð4Þ

The thermal energy model was used to capture the heat transfer in the solid part. For the fluid flow analysis, different turbulent models were used based on fluid properties and flow characteristics. The combustion phenomenon is highly turbulent in characteristics due to high velocity of air and fuel as well as the reaction, which produces large amounts of energy in the form of heat. The cold air flowing outside the heat exchanger made to flow along the surface of small fins where high velocity fields were expected. To capture the turbulence effects in cold air and hot gas, the BSL RS (Baseline Reynolds Stress) turbulence model was used. This sug-

gested model provided good predictions of the characteristics and physics of the most flows relevant to various industries, especially, if the complex flow involved effects of streamlined curvature and a strong swirl component [45]. The K  e model for the water side is considered and predicted boundary layer simulations with good accuracy and was suitable for the water flow under consideration [46]. The steady-state governing equations were discretized using the finite volume method (FVM), and the high-resolution option was used for the advection scheme. The high-resolution option attempted to set the order of the scheme as high as possible automatically while keeping the solution bounded everywhere. The SIMPLE algorithm was used to deal with the coupling between the pressure and velocity [47]. The BSL Reynolds stress of Eq. (5) is described below [42]:

@ðqxÞ x þ @ðU k qxÞ ¼ a3 Pk þ Pxb  b3 qx2 @t k
 @  l  @x þ l þ t3 @xk rx @xk 1 @k @ x þ ð1  F 1 Þ2q r2 x @xk @xk

ð5Þ

where the coefficients a and b of the x-equation, as well as both the turbulent Prandtl numbers r and r2 , were blended between values from the two sets of constants, corresponding to the x-based model constants and the e-based model constants transformed to an xformulation. 2.3. Assumptions The numerical simulation was performed to collect the fluid flow and temperature fields in an integrated heating system that coupled the thermal management for the cabin air and battery for electric vehicles under steady-state operating conditions in cold weather. The governing equations were used for the turbulent flow

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308.0 13.6

307.8 307.6

13.4

307.2 307.0

13.2

Mesh independant

306.8 13.0

306.6 306.4

12.8

306.2

Outlet cold air temperature Pressure drop

306.0

12.6

305.8 305.6

Pressure drop (Pa)

Temperature ( oC)

307.4

6000000

8000000

10000000

12000000

14000000

Number of elements

(a) 298.90

112.7

112.6

Mesh independant

298.86 298.84

112.5

298.82

Outlet water temperature Pressure drop

298.80 298.78 298.76

112.4

Pressure drop (Pa)

Temperature ( oC)

298.88

112.3 6000000

8000000

10000000

12000000

14000000

Number of elements

(b) Fig. 3. Mesh independency results (a) Outlet cold air temperature and pressure drop and (b) Outlet water temperature and pressure drop.

and heat transfer of the heat exchanger with cold airflow rate, hot gas flow rate and water mass flow rate. The cold air flow and hot gas flow were assumed to be incompressible, since the maximum flow velocities within the flow domains range between 0 and 7.5 m/s, which corresponded to subsonic flow. In this case, the Mach number, which was the ratio of the fluid velocity to the speed of sound, was lower than 0.1, resulting in a subsonic flow regime, so compressibility effects could be neglected. In addition, the incompressible flow model considerably lowered the computational load compared to a compressible flow model. The composition of the hot gas was assumed as ideal mixture of the combustion product gases based on a balanced chemical equation. Table 5 showed the ideal mass fraction of the product gases.

Table 5 Ideal mass fraction of products. Product component

Mass fraction

CO2 H2O N2 O2

0.1623 0.0884 0.7391 0.0102

current analysis global length was chosen as characteristic length, which was cube root of the volume.

Re ¼

quD0 l

ð6Þ

3. Data reduction

3.2. Nusselt number

3.1. Reynolds number

The average Nusselt number was determined by Eq. (7) with the average heat transfer coefficient (hc ), the characteristics length (D0 ) and the fluid thermal conductivity (k).

Reynolds number was determined by Eq. (6), using density ðqÞ, mean fluid velocity ðuÞ, characteristics length ðD0 Þ and dynamic viscosity ðlÞ. The integrated geometry under consideration varied significantly in size and shape at many points. Therefore, for

Nuav g ¼

hc D 0 k

ð7Þ

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3.3. Overall heat transfer effectiveness For three fluid heat exchanger with cold air, hot gas and water as the working fluids, the overall heat transfer effectiveness of the heat exchanger was calculated by Eq. (8). Saeid and Seetharamu reported the overall effectiveness of three fluid heat exchanger using the finite element method [48].

e¼

hhin  hhout n  o   1 Min 1; R1 þ R1 hhin  Min hcin ; hwin 2

ð8Þ

where

h¼

T  T cin ; T hin  T cin

R1 ¼

_ cp Þh ðm ; _ cp Þc ðm

R2 ¼

_ cp Þh ðm _ cp Þw ðm

3.4. Percentage of heat transfer rate The percentage heat transfer rate shows the relative heat transfer rate in cold air and water section.

Percentage of heat transfer rate ¼

heat transfer rate for water or cabin air  100 Total heat transfer rate

mass flow rate of 0.010 kg/s. The overall heat transfer rate of the cold air side, the water side and the exhaust gas side were compared as shown in Fig. 4(a) and the average outlet temperature of cold air and water of the integrated heating system were compared with the existed data as shown in Fig. 4(b). All the comparisons showed good agreement within ±9.0% of the existed data and we thus confirmed that the present analysis model was valid. 4.2. Basic performances Fig. 5 showed the average Nusselt number, pressure drop, and heat transfer effectiveness of the integrated heating system for cabin and battery of an electric vehicle with the Reynolds numbers for the cold air and the water, respectively. The average Nusselt numbers were provided as a qualitative measure of the convective heat transfer occurring at the surface due to the ratio of convection heat transfer to fluid conduction heat transfer. Therefore, the high Nusselt number meant the enhancement of the heat transfer owing to the fluid motion. The average Nusselt numbers of the integrated heating system for the cold air side and the water side were increased with the rise of the Reynolds numbers due to the significantly increased wall heat transfer coefficient as shown in Fig. 5(a). The average Nusselt numbers for the cold air side and the water side were increased 28.4% and 9.5%, respectively, with

3.5. Integrated heating system efficiency

¼

Heat transfer rate of cold air and water Total heat transfer rate

4. Results and discussion The heat transfer and flow characteristics of the integrated heating system for cabin and battery of an electric vehicle under the cold weather conditions were numerically investigated with variations of the inlet mass flow rates and inlet temperatures of the cold air and water, respectively. Especially, the inlet cold air mass flow rates and inlet cold air temperatures of the integrated heating system were varied as 0.010, 0.020, 0.030, 0.040 kg/s and 10 °C, 5 °C, 0 °C, respectively. The inlet water mass flow rates and the inlet cold water temperatures of the integrated heating system were varied with 0.010, 0.015, 0.020, 0.025 kg/s and 1 °C, 5 °C, respectively. In addition, the hot gas mass flow rate was varied from 1.25 g/s to 1.85 g/s and the effect on the overall heat transfer rate of the integrated heating system was reported. 4.1. Validation To validate the numerical results with the existed data suggested by Bang et al. [49] using the chemically balanced fuel combustion equations, the overall heat transfer rate with the water mass flow rate of the integrated heating system for the cabin and battery of an electric vehicle were compared. In addition, outlet water and cold air temperature variation with cold air mass rate were also compared. Fig. 4(a) showed comparison of the numerical results with the existed data of Bang et al. [49] for the water mass flow rate ranges from 0.005 kg/s to 0.015 kg/s at the cold air mass flow rate of 0.010 kg/s. Fig. 4(b) showed comparison of the numerical results with the existed data of Bang et al. [49] for the cold air mass flow rate ranges from 0.005 kg/s to 0.015 kg/s at the water

1.8 1.6 1.4

Inlet cold air mass flow rate = 0.010 kg/s Cold air side heat transfer rate (Bang et al.) Cold air side heat transfer rate (Present study) Water side heat transfer rate (Bang et al.) Water side heat transfer rate (Present study) Exhaust gas side heat transfer rate (Bang et al.) Exhaust gas side heat transfer rate (Present study)

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.005

0.010

0.015

Water mass flow rate (kg/s)

(a) 75 70

Inlet water mass flow rate = 0.010 kg/s Outlet cold air temperature (Bang et al.) Outlet cold air temperature (Present study) Outlet water temperature (Bang et al.) Outlet water temperature (Present study)

65

Temperature ( oC)

Integrated heating system efficiency

Overall Heat transfer rate (kW)

2.0

The integrated heating system efficiency of the integrated heating system is the ratio of summation of heat transfer rate of water and cold air to the total heat transfer rate

60 55 50 45 40 35 30 25

0.005

0.010

0.015

Cold air mass flow rate (kg/s)

(b) Fig. 4. (a) Overall heat transfer rate with varying water mass flow rates at the inlet cold air mass flow rate of 0.010 kg/s and (b) Temperature distribution with varying cold air mass flow rates at the inlet water mass flow rate of 0.010 kg/s.

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the increase of the cold air side Reynolds numbers from 15,677 to 72,664 and the water side Reynolds numbers from 4330 to 11,912. In addition, the increasing rate of the average Nusselt number for the cold air side was higher than that of the average Nusselt number for the water side due to larger increase in wall heat transfer coefficient. Fig. 5(a) showed the average Nusselt numbers of the integrated heating system with the Reynolds numbers for the cold air and the water, respectively. Fig. 5(b) showed the pressure drop of the integrated heating system with the Reynolds numbers for

Average Nusselt number for water side Average Nusselt number for cold air side

320

Nuavg

300

280

260

10000

20000

40000

the cold air and the water, respectively. The pressure drop of the integrated heating system increased 870% for the cold air side and 430% for the water side with the rise of the Reynolds numbers. The pressure drop of the integrated heating system for the cold air side showed higher than that for the water side due to the increased resistance with fins. Fig. 5(c) showed the overall heat transfer effectiveness of the integrated heating system with the Reynolds numbers. The overall heat transfer effectiveness for three fluid heat exchanger was defined as the ratio of actual heat transfer to the maximum possible heat transfer based on the given flow and temperature conditions. The minimum overall heat transfer effectiveness of the integrated heating system for the cold air side and the water side was observed as 0.87 and the overall heat transfer effectiveness slightly increased by 2.4% for the cold air side and 2.2% for the water side, respectively, with the rise of Reynolds numbers. This meant that the suggested design of the integrated heating system was effective and the actual heat transfer rate with respect to maximum heat transfer rate was high. In addition, the average overall heat transfer effectiveness of the integrated heating system showed 0.89 for the cold air side and 0.88 for the water side, respectively, at all Reynolds numbers. This means that the suggested design of the integrated heating system could operate effectively for all mass flow rates of the cold air side and water side.

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Cold air temperature distribution Inlet water mass flow rate = 0.010 kg/s Inlet water temperature = 5 oC Inlet cold air temperature = 0oC Cold air mass flow rate 0.01 kg/s Cold air mass flow rate 0.02 kg/s Cold air mass flow rate 0.03 kg/s Cold air mass flow rate 0.04 kg/s

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Water temperature distribution Inlet cold air mass flow rate = 0.010 kg/s Inlet water temperature = 5 oC Inlet cold air temperature = 0 oC Water mass flow rate = 0.010 kg/s Water mass flow rate = 0.015 kg/s Water mass flow rate = 0.020 kg/s Water mass flow rate = 0.025 kg/s

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(c) Fig. 5. Average Nusselt number, pressure drop, and overall heat transfer effectiveness of the integrated heating system for cabin and battery of an electric vehicle with the Reynolds numbers for the cold air and the water (a) Average Nusselt number (b) Pressure drop and (c) Overall heat transfer effectiveness.

-0.01 0.00

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(b) Fig. 6. Temperature distributions of the integrated heating system with the inlet water mass flow rate of 0.010 kg/s, the inlet water temperature of 5 °C, the initial inlet cold air temperature of 0 °C (a) cold air temperatures with the cold air mass flow rates and (b) water temperatures with the water mass flow rates.

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4.3. Heat transfer characteristics Fig. 6 showed the cold air and water temperatures of the integrated heating system for cabin and battery of an electric vehicle at the inlet water mass flow rate of 0.010 kg/s, the inlet water temperature of 5 °C, the inlet cold air temperature of 0 °C. As shown in Fig. 6(a), the cold air temperatures of the integrated heating system were steadily converged for all cold air mass flow rates after the position of 0.22 m of the flow direction because the heat exchange between the hot gas and cold air was reduced with the change of the hot gas flow direction at the exhaust. The cold air temperatures of the integrated heating system were decreased with the increase of the cold air mass flow rates from 0.010 kg/s to 0.040 kg/s at the water mass flow rate of 0.010 kg/s, _ p ) of the cold air increased as the because the heat capacity (mC mass flow rate was increased. The cold air temperatures of the integrated heating system decreased by 42.9% and 22.6%, as the cold air mass flow rate increased from 0.010 kg/s to 0.020 kg/s and from 0.030 kg/s to 0.040 kg/s, respectively. In addition, the cold air temperatures of the integrated heating system at all cold air mass flow rates increased sharply and observed the peak values between 0.175 m and 0.20 m because of the sudden contraction effect of the flow caused by the diameter reduction before the exhaust gas [50]. Fig. 6(a) showed the cold air temperatures of the integrated heating system with the cold air mass flow rates from 0.010 kg/s to 0.040 kg/s.

Fig. 6(b) showed the water temperatures of the integrated heating system with the water mass flow rates from 0.010 kg/s to 0.025 kg/s. The water temperatures of the integrated heating system at all water mass flow rates increased with the flow direction and decreased with the rise of the water mass flow rates from 0.010 kg/s to 0.025 kg/s due to the increase of the heat capacity of the water with the increased water mass flow rate. This resulted in smaller increase of the water temperature. In addition, the water temperature at 0.06 m position of the integrated heating system decreased by 42.3% with the rise of the water mass flow rates from 0.010 kg/s to 0.025 kg/s. Fig. 7 showed the outlet temperatures of the integrated heating system for cabin and battery of an electric vehicle for all water mass flow rates with the variations of the cold air mass flow rates from 0.010 kg/s to 0.040 kg/s at the inlet water temperature of 1.0 °C and the inlet cold air temperature of 10 °C. All outlet temperatures for the water, the cold air, and the exhaust gas of the integrated heating system for all mass flow rates were decreased with the rise of the cold air mass flow rates due to the increased heat capacity with the increased mass flow rate. For same heat transfer rate, with the higher heat capacity temperature variation decreases. The outlet temperatures for the cold air, the exhaust gas, and the water of the tested integrated heating system at the water mass flow rate of 0.010 kg/s decreased by 82.6%, 31.7%, and 21.3%, respectively, with the rise of the cold air flow rates from 0.010 kg/s to 0.040 kg/s. The decreasing rate of the outlet cold air

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Fig. 7. Outlet water, outlet cold air and outlet exhaust temperatures of the integrated heating system for cabin and battery of an electric vehicle for all water mass flow rates with the variatons of the cold air mass flow rates from 0.010 kg/s to 0.040 kg/s at the inlet water temperature of 1.0 °C and the inlet cold air temperature of 10 °C.

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ing system for cabin and battery of an electric vehicle with the variations of the water mass flow rate and the cold air mass flow rate. All outlet temperatures of the integrated heating system decreased with the rise of the cold air mass flow rate and the water mass flow rate because of the improved heat capacity. The heat capacities of the cold air and the water were increased with increase of the mass flow rate of the cold air and water, respectively. As a result, the outlet cold air and the outlet water temperature of the integrated heating system decreased with the increase of the mass flow rate of the cold air in view of same amount of the heat transfer rate. In order to investigate the heat transfer characteristics of the integrated heating system for cabin and battery of an electric vehicle, overall heat transfer rate and efficiency of the integrated heating system with variations of the inlet mass flow rates for the cold air and the water at various inlet water and cold air temperatures were considered. Fig. 9 showed the overall heat transfer rate and the integrated heating system efficiency of the integrated heating system for cabin and battery of an electric vehicle with the variations of the water and the cold air mass flow rate at the various inlet water and cold air temperature. The overall heat transfer rate of the integrated heating system increased with the rise of the water mass flow rates because of increase in the wall heat transfer coefficient. Due to increase in the wall heat transfer, Nusselt number for water side increased considerably which enhanced the heat transfer rate. The overall heat transfer rate of the integrated heating system, at the cold air mass flow rates of 0.010 kg/s and 0.040 kg/s increased

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temperature of the investigated heating system for all water mass flow rates were higher than that of the outlet water temperature with the rise of the cold air mass flow rates due to larger specific heat capacity of the water compared to the cold air. In addition, the outlet water temperature of the tested heating system decreased continuously due to the combined effect of the integrated geometry and the relatively higher specific heat capacity at the higher water mass flow rate. Fig. 8 showed the outlet temperatures of the integrated heating system for cabin and battery of an electric vehicle for all cold air mass flow rates with the variations of the water mass flow rates from 0.010 kg/s to 0.025 kg/s at the inlet water temperature of 5.0 °C and the inlet cold air temperature of 0 °C. All outlet temperatures for the water, the cold air, and the exhaust gas of the integrated heating system for all cold air mass flow rates were decreased with the rise of the water and the cold air mass flow rates due to the enhanced heat capacity with the increased mass flow rates for the cold air side and the water side. The outlet temperatures for the cold air, the exhaust gas, and the water of the integrated heating system at the water mass flow rate of 0.010 kg/s decreased by 65.4%, 29.3%, and 17.6%, respectively, with the rise of the water mass flow rates from 0.010 kg/s to 0.025 kg/s. The decreasing rate of the outlet cold air temperature of the integrated heating system for all cold air mass flow rates were higher than that of the outlet water temperature with the rise of the water mass flow rates due to larger specific heat capacity of the water compared to the cold air. Based on Figs. 7 and 8, the similar results were observed for the outlet temperatures of the integrated heat-

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Fig. 8. Outlet water, outlet cold air and outlet exhaust temperatures of the integrated heating system for cabin and battery of an electric vehicle for all cold air mass flow rates with the variatons of the water mass flow rates from 0.010 kg/s to 0.025 kg/s at the inlet water temperature of 5.0 °C and the inlet cold air temperature of 0 °C.

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Fig. 9. Overall heat transfer rate and efficiency of the integrated heating system for cabin and battery of an electric vehicle with the variations of the cold air and the water mass flow rate at the various inlet water and cold air temperature.

by 1.6% and 1.1%, respectively, with rise of the water mass flow rate from 0.010 kg/s to 0.025 kg/s, at inlet water temperature of 1 °C and inlet cold air temperature of 10 °C. The overall heat transfer rate of the integrated heating system was increased with the increase of the mass flow rate of water, but when the cold air mass flow rate was increased, the overall heat transfer rate of the integrated heating system was decreased. The integrated heating system under consideration is a three-fluid heat exchanger with hot gas, cold air and water. The heat flow rates are interdependent and influence the overall heat transfer rate. The behavior of decrease of overall heat transfer rate as the mass flow rate of the cold air increase is observed and the reason for this behavior could be due to low specific heat of the air as compared to water which affects the overall heat transfer rate in three-fluid heat exchanger. Furthermore, the overall heat transfer rate increased with the rise of heat capacity of the water and decreased with the rise of the heat capacity of cold air. The overall heat transfer rate increased 1.6% and 1.1% for cold air mass flow rates of 0.010 kg/s and 0.040 kg/s, respectively, at the inlet water temperature of 1 °C and inlet cold air temperature of 10 °C. In addition, the variation in overall heat transfer rate at low mass flow rate was maximum and at high mass flow rate was minimum due to high sensitivity for heat transfer rate at low mass flow rates of the cold air and the water than at the high mass flow rates of the cold air and the water. The integrated heating system efficiency could be defined as the ratio of summation of heat transfer rate of water and cold air to the total heat transfer rate. The integrated heating system efficiency of the integrated heating system increased continuously with the increase of the cold air and the water mass flow rate. The inte-

grated heating system efficiency of the integrated heating system at the water mass flow rates of 0.010 kg/s and 0.025 kg/s increased by 3.3% and 2.1%, respectively, with the rise of the cold air mass flow rate from 0.010 kg/s to 0.040 kg/s at inlet water temperature of 1 °C and inlet cold air temperature of 10 °C. This was because of the increase in the Nusselt numbers of the integrated heating system with the increased mass flow rate for the cold air side and the water side. For winter condition with 10 °C inlet cold air and 1.0 °C inlet water temperature, the maximum cold air temperature of 37.8 °C and maximum water temperature of 21.6 °C were observed at cold air mass flow rate of 0.010 kg/s and water mass flow rate of 0.010 kg/s, suggesting sufficient temperature rise for heating of cabin and battery of an electric vehicle. For early spring condition with 0 °C inlet cold air condition, the maximum cold air temperature of 46.3 °C and maximum water temperature of 25.9 °C were observed at cold air mass flow rate of 0.010 kg/s and water mass flow rate of 0.010 kg/s, suggesting sufficient temperature rise for heating of cabin and battery of an electric vehicle. Based on the heat transfer rate and outlet temperatures, for the winter condition, the cold air mass flow rate of 0.01 kg/s and water mass flow rate of 0.01 kg/s are suggested. In addition, for the early spring condition, the cold air mass flow rate of 0.01 kg/s and water mass flow rate of 0.015 kg/s could be used. Fig. 10 showed the percentage of heat transfer rate of the integrated heating system for cabin and battery of an electric vehicle with the variations of the cold air and the water mass flow rate at the various inlet water and cold air temperature. For all the inlet water and cold air temperature cases, when the cold air mass flow rates was increased from the 0.010 kg/s to 0.040 kg/s, the percent-

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Fig. 10. Percentage of heat transfer rate of the integrated heating system for cabin and battery of an electric vehicle with the variations of the cold air and the water mass flow rate at the various inlet water and cold air temperature.

age of heat transfer rate increased for the cold air and decreased for the water. For example, the percentage of heat transfer rate increased from 32.2% to 46.2% for cold air and decreased from 55.6% to 44.8% when the cold air mass flow rate increased from 0.010 kg/s to 0.040 kg/s at inlet water temperature of 1 °C and inlet cold air temperature of 10 °C. In three-fluid heat exchanger case, increasing flow rate of one fluid will reduce the heat transfer rate of the other side [51]. Therefore, in this case, increasing the flow rate of the water reduced the heat transfer rate of the cold air side. The increase of the water mass flow rate increased the Nusselt number for water side which increased the heat transfer rate for water side and this caused reduction in heat transfer rate of the cold air side. Similarly, increase of cold air mass flow rate increased the Nusselt number for cold air side, which increased the heat transfer rate for cold air side and this caused reduction in heat transfer rate of water side. The variation at low mass flow rate of the cold air and the water was higher than the large mass flow rate of the cold air and the water. For example, percentage of heat transfer rate increased by 14.0% and 12.5% for the cold air when cold air mass flow rate increased from 0.010 kg/s to 0.040 kg/s at inlet water temperature of 1 °C and inlet cold air temperature of 10 °C, at water mass flow rate of 0.010 kg/s and 0.025 kg/s, respectively. It could be clearly observed that the percentage variation was large for the cold air mass flow rate of 0.010 kg/s and the water mass flow rate of 0.010 kg/s compared to other mass flow rates in all the cases. The observed phenomenon was due to high sensitivity

for heat transfer rate at low mass flow rates of the cold air and the water than at the large mass flow rates of the cold air and the water. As the integrated heating system was tested with wide range of inlet mass flow rates and inlet temperatures with the similar trends of the heat transfer rate variation, the flow rates could be adjusted freely according to the demand for the heating of cabin and battery of an electric vehicle. Fig. 11 showed overall heat transfer rate of the integrated heating system for cabin and battery of an electric vehicle with variations of the hot gas mass flow rate at inlet water temperature of 1.0 °C and inlet cold air temperature of 10 °C with cold air mass flow rate of 0.010 kg/s and water mass flow rate of 0.010 kg/s. The overall heat transfer rate of the integrated heating system increased 47.8% when the hot gas mass flow rate was increased from 1.25 g/s to 1.85 g/s. The overall heat transfer rate of 2kW could be achieved for hot gas mass flow rate of higher than 1.70 g/s. The heat transfer rate could be adjusted based on the demand by changing the hot gas mass flow rate. The variation of heat transfer rate with mass flow rates of hot gas, cold air water provides a substantial control to accommodate demand and supply conditions. The general need for cabin heating in hybrid and fully electric vehicle vary from 2.0 kW to 10 kW based on the size of the vehicle as well as operating ambient condition. The development of integrated heat exchanger is an innovative step towards solving thermal management issue of electric vehicles operating at low ambient temperature and to enhance the travel range. Practically, the use of integrated heating system

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Overall heat transfer rate (kW)

92

2.5

Overall heat transfer rate Inlet water mass flow rate = 0.010 kg/s Inlet water mass flow rate = 0.010 kg/s Inlet water temperature = 1oC Inle cold air temperature = -10 oC

2.0 1.5 1.0 0.5

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Hot gas mass flow rate (g/s) Fig. 11. Overall heat transfer rate of the integrated heating system for cabin and battery of an electric vehicle with the variations of the hot gas mass flow rate at the inlet water temperature of 1.0 °C and the inlet cold air temperature of 10 °C with cold air mass flow rate of 0.010 kg/s and water mass flow rate of 0.010 kg/s.

could influence the travel range of EVs as high as 15–20% enhancement. 4.4. Flow visualization Fig. 12 showed the flow characteristics of the integrated heating system for cabin and battery of an electric vehicle with the cold air mass flow rate of 0.010 kg/s and the cold air temperature of 0 °C and the water mass flow rate of 0.010 kg/s and the water temper-

ature of 1.0 °C. Fig. 12(a) and (b) showed the flow patterns of the hot gas flow of the integrated heating system. The hot gas velocity near the baffles of the AHE (upper part) for the tested heating system was very high and many recirculation zones were created due to the cross baffle arrangements. The hot gas of the CHE (lower part) for the integrated heating system was flowing uniformly along the flow direction due to the parallel fin arrangement as shown in Fig. 12(b), but the velocity varied due to the cross arrangement of the water tube with the flow direction. Fig. 12(c) showed the flow patterns of the cold air of the integrated heating system. The wake flow at all corners was observed due to the low pressure regions [52] and then velocity increase was observed at smaller area. Fig. 12(d) showed the flow patterns of the water tube of the integrated heating system. The velocity distribution in the water tube showed relatively uniform flow pattern but the high velocity regions at the curvature of tube were observed because the flow direction was changed. The result section focused on the temperature distribution along with overall heat exchanger performance, Nusselt number variation with Reynolds number, heat transfer rate, pressure drop and velocity distribution. The mass flow rate variation significantly affected the temperature distribution of cold air section and water section. Although the overall heat exchanger effectiveness and Nusselt number increased as cold air and water mass flow rates were increased, the pressure drop increased. The integrated heating system is complex geometry involving number of fins, baffles, tube as well flow partitions. The future studies could include the optimization based on the geometry considering the tube dimension, fin as well as baffle dimension, overall system dimensions as design variables to optimize heat capacity and weight reduction of the integrated heating system.

Fig. 12. Flow characteristics of the integrated heating system for cabin and battery of an electric vehicle with the cold air mass flow rate of 0.010 kg/s and the cold air temperature of 0 °C and the water mass flow rate of 0.010 kg/s and the water temperature of 1.0 °C (a) Hot gas flow (top view), (b) Hot gas flow (side view), (c) Cold air flow (side view) and (d) Water flow (top view).

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5. Conclusions

References

This study investigated the heat transfer characteristics of the integrated heating system for cabin and battery of an electric vehicle under the cold weather conditions, including variations in the inlet mass flow rates and inlet temperatures of the cold air and the water, respectively. The integrated heating system was suggested to provide the dual role as the cabin air heater and the battery thermal management to maintain the cabin air temperature and battery operation of an electric vehicle under cold weather. The model consisted of a burner for combustion, two integrated heat exchangers and a fan. The numerical model was developed and validated within ±9% of the previously published results, for the water mass flow rates of 0.010–0.015 kg/s at the cold air mass flow rates of 0.01 kg/s. The parametric effects on the temperatures, heat transfer rate, and flow characteristics of the cold air and the water were discussed. In order to investigate the performance of the integrated heating system, the Nusselt number, pressure drop and overall heat exchanger effectiveness variation with Reynolds number were reported. The Nusselt number, the pressure drop and the overall heat exchanger effectiveness increased continuously with the rise in the Reynolds number. The overall heat transfer effectiveness of the integrated heating system showed 0.89 for the cold air side and 0.88 for the water side, respectively, at all Reynolds numbers, suggesting that the design of the integrated heating system could operate effectively for all mass flow rates of the cold air side and water side. The outlet temperatures of the cold air, the water and the exhaust gas of the integrated heating system decreased continuously with the rise of the cold air mass flow rate from 0.010 kg/s to 0.040 kg/s and with the rise of the water mass flow rate from 0.010 kg/s to 0.025 kg/s, because as the mass flow rate increased, the heat capacity increased, resulting in smaller increase in cold air and water temperatures. Due to integrated 3-fluid heat exchanger system, as the cold air mass flow rate increased the heat transfer rate of the cold air increased and the water heat transfer rate decreased. Similarly, as the water mass flow rate increased, the heat transfer rate of water increased and cold air mass flow rate increased. The efficiency of the integrated heating system increased continuously with the increase of cold air and water mass flow rate due to the increase in the Nusselt numbers of the integrated heating system with the increased mass flow rate for the cold air side and the water side. Finally, flow visualization of the integrated heating system was discussed to understand the effect of the complex geometry on the flow characteristics. The heat transfer rate could be adjusted based on the demand by changing mass flow rates of the hot gas, the cold air and the water. Based on the results observed for the integrated heating system considering performance, heating transfer rate, temperature distribution and flow characteristics for the integrated heating system, it is suitable for use as a cabin air heating system coupled with a battery heating system in electric vehicles operating at cold weather condition particularly in winter and early spring condition.

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Declaration of interest We confirm that the manuscript has been read and approved by all named authors. We wish to confirm that there are no known conflicts of interest associated with this publication. Acknowledgements This work was supported by the Dong-A University research fund.

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