Transient cooling effect analyses for a permanent-magnet synchronous motor with phase-change-material packaging

Applied Thermal Engineering 109 (2016) 251–260

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Transient cooling effect analyses for a permanent-magnet synchronous motor with phase-change-material packaging Shengnan Wang a, Yunhua Li a, Yun-Ze Li b,⇑, Jixiang Wang b, Xi Xiao c, Wei Guo a a

School of Automation Science and Electrical Engineering, Beihang University, Beijing 100191, China School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China c Aviation Key Laboratory of Science and Technology on Aero Electromechanical System Integration, Jincheng Nanjing Electrical and Hydraulic Engineering Research Center of AVIC, Nanjing 211106, China b

h i g h l i g h t s
A paraffin based casing for the cooling of actuator motor was proposed.
The working time of the motor can be prolonged with this paraffin-based casing.
The motor’s peak temperature can be decreased during the period working modes.
Bigger quality and higher melting rate of paraffin lead to better cooling effect.
The melting point of the paraffin affects the thermal control performance greatly.

a r t i c l e

i n f o

Article history: Received 28 February 2016 Revised 13 July 2016 Accepted 4 August 2016 Available online 5 August 2016 Keywords: Permanent-magnet synchronous motor Phase change material Thermal management More electric aircraft

a b s t r a c t Permanent-magnet synchronous motors (PMSMs) are widely involved in more-electric aircrafts and allelectric aircrafts. But the cooling strategy of the PMSM is still a challenge for the designers. This paper proposed a novel thermal management approach with phase change material (PCM) for a PMSM applied in the actuator systems of aircraft. A simplified 3D model of the PMSM with a special casing packaged by paraffin-PCM was built. With the finite element method, the impact of paraffin cavity configurations and the paraffin types on the transient cooling effect of the casings have been simulated and analyzed under conditions of various heat load duty cycles and different ambient temperatures. The results suggest that, by replacing the convectional motor casing with this paraffin-based enclosure, the effective time for the PMSM temperature control could be prolonged by approximate 32.7% when the motor works under a continuous mode, and the peak temperature of the PMSM could be decreased evidently when the PMSM operates under a periodic mode. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction With the development of more electric aircrafts (MEAs) and all electric aircrafts (AEAs), most of the aerospace systems are undergoing a long-term transition from using mechanical, hydraulic, and pneumatic power systems toward globally optimized electrical systems [1]. And the electric motors make the ultimate goal to be achieved by converting electrical power to mechanical work to drive actuators, pumps, compressors and other subsystems at variable speeds [2–4]. Hundreds of electric motors are demanded for a large MEA, and hundreds of kilowatt electric power is required to drive the motors. However, the output power can never ⇑ Corresponding author. E-mail address: [email protected] (Y.-Z. Li). http://dx.doi.org/10.1016/j.applthermaleng.2016.08.036 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

equal the input power, for there are always losses. A larger amount of heat will be unavoidably generated inside the motor with an increasing requirement for the output power [5,6]. Most of the motors on aircraft can no longer be kept at a safe temperature level by only natural convection cooling [7], such as the permanentmagnet synchronous motors (PMSMs) in actuator systems [8,9]. There is a contradiction between the large heat generation and the insufficient cooling capacity of the PMSMs during the operating time. Therefore, how to balance the thermal management requirement and the limited heat releasing ability of the motor is an imperative problem to be solved. There are various approaches to realizing the motor cooling. For instance, the simplest one is self-cooling by natural convection with external fins incorporated in the casing, and the cooling effect could be enhanced by a fan mounted on one end of the shaft

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Nomenclature g c h k l m n nr q x, y, z C Gr L Nu Ni, Nj P Pr Q Re R T DT V

gravity coefficient specific heat capacity, J/kg K heat convection coefficient, W/m2 K thermal conductivity, W/m K height of the casing, m mass of paraffin, kg normal direction of surface rotational speed of the rotor, r/min heat flux into the boundary, W/m2 component in a Cartesian coordinate system heat capacity, J/K Grashotf number heat latent of the paraffin, J/kg Nusselt number interpolation function heat power, W Prandtl number heat, J Reynolds number heat resistant, K/W temperature, K temperature difference, K volume, m3

Greek symbols a expansion coefficient of air b surface roughness factor d length of air gap, m e emissivity factor

[10,11]. This method is suitable to the motors with low power density for its low heat dissipating efficiency. Another kind of motor thermal management tactic is the liquid-cooling. Two working fluids, water [12,13] and oil [14,15], were generally used in the previous research. For the first one, sophisticated design with internal water channels inserted in the casings is a major work. It can provide a long time constant and high thermal overload capacity with high efficient fluidic path arrangement. Moreover, the cooling circle can be shared with other collaborative devices such as inverter and braking resistor. The oil-cooling scheme, including oil bath and oil injection which are usually used for the high-power motors in aircraft, performs better for the higher thermal capacity of oil compared with that of water. These liquid-cooling methods share a common disadvantage that external fluid circle system including pump package, heat dissipation unit, tube system, etc. will increase the whole system’s weight and size, and decrease the reliability and efficiency. Therefore, self-cooling or air-cooling is still the major thermal management method for the PMSMs in actuator systems. Meanwhile, in consideration of the intermittent operating mode of the PMSMs, the phase change heat storage could be used to improve the cooling performance of the motors. The phase change material (PCM) has been widely applied in heat storage and thermal management systems because it provides a high energy storage density. The paraffin-PCM, for an example, absorbs approximately 200 J/kg–290 kJ/kg of heat if it undergoes a melting process [16,17]. High amount of heat absorbed by the paraffin can be released to the surroundings in a cooling process starting at the PCM’s crystallization temperature. What is more, the latent heat storage has the capacity to store heat of fusion at an almost constant temperature corresponding to the phase transition temperature of the PCM. The paraffin-PCM with specific characteristics, such as melting temperature range, volume expansibility, and density, can be made to meet various needs by blended

X C W

heat storage proportion,% liquid fraction of paraffin rotor peripheral speed, rad/min viscosity of air, m2/s density, kg/m3 Boltzmann constant operating time of the motor, s volume of each element boundary surfaces of elements Production power density, W/m3

Subscript 0 ave c e eff h i, j init load off m1 m2 para r s H_ave L_ave

ambient average value casing partitioned finite element equivalent value heat convection number of node initial time operating state stop state solidification point of the paraffin melting point of the paraffin paraffin rotor stator average value during the heating time average value during the interrupt time

n

g x m q r s

different raw materials and accessory materials [18]. However, its low thermal conductivity sharply restricts the efficient charging and discharging process of latent heat energy. To solve this problem, some heat transfer and enhancement techniques can be employed by adding fins, high conductivity metal or graphite particles into the PCMs [19–22]. Many investigators have applied the PCM to the airconditioning and electronic devices thermal controlling systems. Turnpenny et al. [23] first published their study about the cooling of buildings using heat pipes embedded in PCM (Na2SO410H2O) storage unit in 2000. In the next year, they [24] installed a prototype free cooling system to prevent a typical office building from being overheating in summer. Jankowski et al. [25] had discussed the PCM applications in vehicle component thermal buffering. Tan et al. [26] designed a helmet cooling system with PCM to store the heat produced by the wearer head, and the helmet could provide comfortable cooling for up to 2 h when the PCM was completely melted. They [27] also studied the cooling of mobile electronic devices (heat power from 4 W to 16 W) with a heat storage unit filled with n-eicosane. Kandasamy et al. [28] investigated a PCM-based heat sink experimentally and numerically for the thermal management of electronic devices with the input power ranging from 2 W to 6 W. Several researchers have discussed the temperature control effect of high power devices with PCMs. Lu [29] studied the prospect of high power electronic packages with phase change cooling by numerical method. In addition, Bellettre et al. [30] tried to put paraffin around the windings in the fullyenclosed motor to cool the hot spot in 1997. He gave some nodal type models to simulate the transient phase change cooling performance and a good temperature control result was obtained. But it is hard to hold the liquid paraffin in a fixed position relative to the windings inside the motor, and how to ensure that the paraffin is reusable after its solidification process remains to be solved.

S. Wang et al. / Applied Thermal Engineering 109 (2016) 251–260

In order to find a thermal management strategy for the natural convection cooling PMSMs on aircraft, a novel closed type motor casing packaged with paraffin-PCM is designed. Preliminary numerical analysis with node-network method with a 2D model was done in our previous research [31]. In the present paper, an improved 3D model with finite element method was built. A more accurate and detailed simulation study about the transient cooling effect of this paraffin-based cooling approach was carried out under conditions of various work duty cycles and different ambient temperatures. The results indicate that the extremely high temperature of the PMSM could be decreased and the operating time of the motor could be prolonged with this paraffin-PCM based thermal control method.

253

the complex combined stator is replaced by a hollow copper cylinder as an equivalent heat source (U95 mm  U45 mm  60 mm). A Silicon-iron cylinder (U34.4 mm  U24 mm  60 mm) is built as the rotor which is bound around a carbon steel shaft (U24 mm  165 mm). Comparative analysis can be conducted with this 3D model by replacing the paraffin blocks with solid aluminum alloy. Specifications of the 3D model are listed in Table 1. 2.2. PCM-based thermal control idea The concept of PCM-based thermal managing approach with a casing packaged by PCM for the PMSM, as well as the traditional natural convection cooling method without PCM, is shown in Fig. 1.

2. PMSM configurations and their thermal control idea 2.1. PMSM configurations The diagram of a traditional PMSM is shown on the left side of Fig. 2, which is mainly composed of a stator, several windings, a rotor, a shaft and a casing with fins. On the basis of a real PMSM to be designed for rudder control, a simplified 3D motor model is drawn on the right side of Fig. 2, which is constituted by a main cylinder-shaped aluminum alloy casing with fins and cavities for PCM (paraffin), a stator, a rotor, a long shaft and two end covers. The bearings are omitted for simplicity, and their effect on the heat transmission between the shaft and the two end covers could be exerted by imposing thermal contact resistance on the interfaces. The stator is interference fitted with the main casing, and most of the heat generated inside the motor is translated out of the casing through the junction surface of them. Because the thermal managing ability of the casings is the main consideration here,

2.2.1. Traditional natural convection cooling approach and its issues For a conventional natural convection cooling PMSM which works in an intermittent mode, a large amount of heat is generated in a short electrifying time of sload, followed by a long interrupt time of soff when the power is shut off. The casing with fins is the only cooling device of the PMSM. All heat inside the motor needs to be released directly through the casing to the ambient and the motor suffers a high average temperature T H av e during sload. Since no more heat is produced in soff, the average temperature of the motor T L av e is relatively lower than T H av e . Under the precondition of keeping the temperature of motor under the heat resistant temperature, when the cooling capacity of the casing is designed to meet the high heat dissipating requirement in sload, the size of the motor will be too large to be acceptable for the application in aircraft. In contrary, if the heat dissipation ability of the casing only matches the period-average heat power, the motor will be overheated easily.

Fig. 1. Heat dissipation and temperature variation curves of PMSMs cooled by PCM packaged casing and conventional casing without PCM respectively.

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Fig. 2. Structural diagrams of a traditional PMSM and simplified 3D model of PMSM with paraffin as PCM for simulation.

Table 1 Parameters of the PMSM model. Property

Value

Rated input power Rated output power Heat power Rated speed Stator OD Rotor OD Active stack length Casing OD Casing length Fin sizes

2800 W 2530 W 270 W 10,700 rpm 95 mm 44 mm 60 mm 114 mm 145 mm 1.6 mm  2 mm  4.5 mm

2.2.2. PCM-based thermal control idea with a PCM packaging casing When the traditional casing of the PMSM is substituted for a new one packaged with PCM, which is shown in the lower block in Fig. 1, the temperature condition of the PMSM will be improved significantly. On one hand, a large part of heat generated inside the motor is absorbed by the PCM as long as the temperature of the PCM reaches to the melting point, which assists the fin surfaces in easing the heat dissipating burden during sload, thereby keeping the motor from overheating. The heat stored by the PCM can be released to surroundings through the casing when the motor stops. On the other hand, the temperature of the motor could be controlled to a range around the phase change temperature of the PCM which is lower than T H av e and higher than T L av e . Above all, the casing with PCM has superiority over the conventional casing in adjusting the quantity of heat dissipation temporally and decreasing the peak temperature of motor spatially during the working periods. Furthermore, the increased heat managing ability can help to reduce the cooling surfaces and decrease the size of motors used in aircraft. In addition, the phase change process of the PCM occurs in a confined space of the casing, so that the PCM could be reused when the motor operates under a periodic work mode. 2.3. Issues of the PCM-based cooling approach The key component of the new casing is the paraffin-PCM sealed in cavities with two end covers. It should be emphasized that enough distance must be left circumferentially between the cavities, otherwise the paraffin circle would become insulation, blocking heat dissipating path and causing an excessively high temperature inside the motor fast. One of the problems about the design of paraffin-PCM packaged casing is how to ensure a long duration of effective thermal control for the motor under a continuous operating condition. When the paraffin is melted completely, it can no longer maintain its temperature in a limited range with the uninterrupted high-density

energy input. For a given weight of paraffin in the casing, the time within which the paraffin can be efficiently exploited to attain the thermal management objective depends on the amount of heat produced by the motor and the cooling conditions outside the casing. The cooling period can be lengthened by using a larger amount of paraffin with a higher latent heat, which will result in a bigger size of the motor in turn. Another problem is the low heat conductivity of the paraffinPCM. A higher thermal absorbing and solidifying rate will benefit a more effective and recyclable application of the paraffin in the motor cooling. In this paper, several aluminum alloy sheets are added into the paraffin blocks to enhance the heat transfer, which, in turn, results in a decrease of the volume of paraffin. 3. Numerical analysis 3.1. Finite element model For the transient temperature of a 3D PMSM model, the nonstationary partial differential equation of heat conduction in Cartesian coordinates can be expressed by Eq.(1).

qc




 @T @ @T @ @T @ @T þ þ þW kx ky kz ¼ @ s @x @x @y @y @z @z

ð1Þ

The boundary conditions in solving Eq. (1) is given by Eq. (2), including the natural convection heat transfer with the air, the radiation heat transfer with the surroundings and the heat conduction between two different materials on the boundaries.

8 @T  k ¼ hðT C  T 0 Þ > > < @nC ¼ erðT 4C  T 40 Þ k @T @n C >     > : k @T ¼ k @T @n C1

ð2Þ

@n C2

The equivalent integration for differential equation of Eqs. (1) and (2) is given by Eq. (3).

 


 @T @ @T @ @T @ @T kx  ky  kz  W dX u qc  @ s @x @x @y @y @z @z X
 Z @T @T @T nx þ ky ny þ kz nz  q þ u1 kx @x @y @z C   @T @T @T nx þ ky ny þ kz nz  hðT 0  TÞ þ u2 kx @x @y @z   @T @T @T nx þ ky ny þ kz nz  erðT 40  T 4 Þ dC ¼ 0 þ u3 kx @x @y @z

Z

ð3Þ

where u, u1, u2 and u3 are arbitrary functions. Let u = u1 = u2 = u3 = dT with Galerkin’s method, and Eq.(3) is translated to a new equation:

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S. Wang et al. / Applied Thermal Engineering 109 (2016) 251–260



 @T @ @T @ @T @ @T dT qc  kx  ky  kz  dT W dX @ s @x @x @y @y @z @z X Z 4 4  dT½q þ hðT 0  TÞ þ erðT 0  T ÞdC ¼ 0: ð4Þ

Z








C

When the finite element method is adopted, the whole model of PMSM is partitioned to numerous small elements which are composed of several nodes, lines and surfaces. An unstructured mesh profile of 2,042,883 elements was generated using the ICEM CFD software with tetra/mixed mesh types including 2D elements of TRI_3 and 3D elements of TETRA_4 and Penta_6 in this paper. The temperature of each element is calculated using interpolation method with the temperatures of the nodes.

Tðx; y; z; sÞ ¼

X Ni ðx; y; zÞT i ðsÞ

ð5Þ

e

The finite equation of temperature field generally takes the form of

C

dT þ KT ¼ P; ds

ð6Þ

where C is the heat capacity matrix, K is the heat conduction matrix, and P is the temperature load matrix, and elements of the matrixes are determined by the following equations.

P e P e 8 K ¼ K þ e Hij > < ij P e ij C ij ¼ e C eij > P P P P : Pi ¼ e Peqi þ e PeWi þ e PeHi þ e P eRi

ð7Þ

All of the above parameters are defined by the following interpolation integrals in the volumes X or on the surfaces C of each element:

8 R  @Ni @Nj e @N i @Nj @Ni > K ¼ > ij Xe kx @x @x þ ky @y @y þ kz @z > > > R > > > Heij ¼ Ce hN i Nj dC > > R > > > C eij ¼ e qcN i N j dX > X < R PeWi ¼ Xe WNi dX > > R > e > > > Pqi ¼ Ce qN i dC > R > e > > > PHi ¼ Ce hT 0 Ni dC > > > R : e PRi ¼ Ce erðT 4  T 40 ÞN i dC

@N j @z



cooling approach can be approved by comparing the peak temperature and the effective thermal control duration time of the paraffin-based casing with those of the traditional one under the same boundary conditions. We assume that the air between the stator and rotor is motionless. The equivalent thermal conductivity keff of the air gap is estimated as 0.082 W/m2 K during the motor’s operating time and 0.03 W/m2 K when the motor stops [33]. The radiation inside the motor is ignored. Taking the thermal resistance induced by the bearings into consideration, we set the contact resistance between the shaft and end covers to 0.16 m2 K/W. The thermal contact resistance between the stator and the casing, as well as that between the PM and the yoke, is so small that it can be neglected because they are interference fitted with each other. As for the boundary conditions outside the casing, radiation and natural convection are involved in the simulation. The fins’ heat dissipation efficiency is set to 1[31]. The surface emissivity is set to 0.7 [34]. The average heat convection efficiency hav e on the surfaces of fins and covers is calculated as 7.36 W/m2 with the method referred in [33]. 3.3. Supporting models In the main cylindrical casing, the heat stored by paraffin can be calculated with Eq. (9).

Q para

dX

8RT > > > T init mcpara dT < R T m1 RT L ¼ mcpara dT þ T m1 T m2 T dT T init m1 > >RT R > T : m1 mc dT þ L þ mcpara dT para T m2 T init

T < T m1 T m1 < T < T m2

ð9Þ

T > T m2

The liquid fraction g of paraffin can be defined by Eq. (10).

:

ð8Þ

With all the equations above, we conducted the temperature field simulating of the motor with Fluent (pressure-based solver), a kind of numerical analysis software based on the computational fluid dynamics. And the Tecplot software was employed to visualize all those temperature contours in the post-processing of the results.

g¼

8 > <0 > :

T < T m1

TT m1 T m2 T m1

T m1 < T < T m2

1

T > T m2

ð10Þ

The average heat storage ability of the paraffin during its melting process is demonstrated by a ratio defined in Eq. (11). In our paper, with the same heat power generated inside the motor, the melting time of paraffin can be estimated by the simulation, while the mass and latent heat of paraffin are known as long as the cavity structure of the casing is determined. The mass and the melting time of paraffin have opposite effects on the ratio.

n¼

mL Vs  W  s

ð11Þ

3.2. Boundary conditions The model motor of this 3D PMSM is designed with insulation material of class E whose highest heat-resistance temperature is 393.15 K [32]. The highest temperature on inner side of the casing beside the stator is selected as the monitoring spot to evaluate the thermal management ability of the casing. Based on the experience of temperature rising tests with natural convection cooling PMSMs in such a scale, the biggest temperature difference between the casing and windings is around 15–20 K. In this paper, the paraffin packaged casing is designed to keep the monitored temperature below 373.15 K, so that the highest temperature of the windings will not exceed 395.15 K. An approximate heat generating process of the PMSM is simulated by imposing a uniform heat load on the simplified stator spatially. The heat will be transferred from the stator and rotor to the outside through the casing. The feasibility of this paraffin-based

3.4. Simulation cases setting Fig. 3 shows the cross-sectional view of the casing model. Twelve paraffin cavities were built in the casing and the heat transmission rate of the paraffin can be enhanced in different degrees by inserting aluminum sheets in them. Four configurations were studied in this paper: (a) N = 0 and d = 0; (b) N = 2 and d = 1 mm; (c) N = 2 and d = 0.5 mm; (d) N = 5 and d = 0.5 mm. In the following sections, the different casings are named as Case1, Case2, Case3 and Case4 corresponding to the four motor enclosures from (a) to (d), and the one without paraffin cavity is called Case0. Simulations with the above cases have been conducted under both continuous and periodic heat load modes. The thermal properties of materials and boundary conditions involved in this paper are listed in Tables 2 and 3 respectively.

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S. Wang et al. / Applied Thermal Engineering 109 (2016) 251–260 Table 3 Working and boundary conditions involved in the simulation [21,34]. Project

Parameter

Duty cycle (sload: sload+soff)

Continuous load 5000 s 0.33 (300 s:900 s) 0.5 (450 s:950 s) 0.67 (600 s:900 s) 0.83 (750 s:900 s) 7.36 W/m2 270 W 0.7 298.15–353.15 K

Heat transfer coefficient Heat generation power Surface emissivity (Mechanical polishing [34]) Environment temperature

Fig. 3. Cross-sectional view of motor casing: number N and thickness d of aluminum sheets can be changed in paraffin cavities.

4. Results and discussion In this part, the impact of paraffin cavity structures on the thermal management performance of the paraffin-based casing is shown from Figs. 4–7. The influence of paraffin type on the cooling effect of the casing is illustrated in Fig. 8. The temperature control performances of the casings under the conditions of different ambient temperatures and various heat load duty cycles are shown from Figs. 9–11. For all the followed simulations, the cooling ways on the outside surfaces of the motor include radiation (e = 0.7) and natural convection (h = 7.36 W/m2 K). All cases are analyzed with a heating power of 270 W and the initial temperatures are the same with the ambient temperatures. The other conditions such as casing configurations, surrounding temperatures, paraffin types, and working duties have been set specifically according to each case in the following parts. 4.1. Impact of paraffin cavity configurations Fig. 4 presents the effect of paraffin cavity configurations on maximum casing temperature (MCT) with paraffin RT90 during a continuous 5000 s heat loading time. Starting with the initial temperature of 298.15 K, which was the same as the boundary temperature, the MCT of Case0 without paraffin reached to the limit temperature (LT, 373.15 K) rapidly after 1070 s which is regarded as the duration of work time (DWT). And the temperature still rose until a steady value was reached. Similarly, the MCTs of casings with paraffin increased fast at first, but the rates became smaller

Fig. 4. Effect of paraffin cavity configurations on maximum casing temperature with paraffin RT90.

when the temperatures of paraffin reached their melting points. When the paraffin was melted completely, the temperatures of the casings increased quickly again to their steady values which were a little higher than that of Case0. Results indicate that the working time could be prolonged with the paraffin based casings compared with Case0 without paraffin. Among all the casings with paraffin, Case4 had the lowest MCT during the melting process for its strongest heat transfer ability of the paraffin cavity. In Fig. 5, the proportions of heat stored by paraffin and those dissipated through the fins to surroundings during the melting progress of paraffin are drawn in pies. Compared with Case1, the heat storage proportion of paraffin in Case2 was reduced because of the small heat resistance of the thick sheets through which more heat was conducted to the fin. And the proportions increased when the casings of Case3 and Case4 were used respectively for the utilization of more thin sheets to enhance the heat absorbing rate of paraffin. Correspondingly, detailed volumes and melting time for the four casings are presented in Fig. 6. The maximum volume of

Table 2 Thermal properties of materials involved in the simulation [21,30]. Density (kg/m3)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

Latent heat (kJ/kg)

Viscosity (kg/m s)

Melting temperature (K) Tm1

Paraffin type

900

0.2

2100

290

0.0135

Air Aluminum alloy Copper Silicon (3%) iron Carbon-steel

1.225 2719 8930 7650 7790

0.082/0.03 202.4 398 27 43.2

1006 871 386 459 470

– – – – –

2.32  105 – – – –

(RT60) (RT70) (RT80) (RT90) – – – – –

Tm2 332.15 342.15 352.15 362.15

334.15 344.15 354.15 364.15

S. Wang et al. / Applied Thermal Engineering 109 (2016) 251–260

257

Fig. 5. Heat storage and dissipation proportions during melting progress of paraffin with different cases.

paraffin blocks. The paraffin at the middle position of the casing was melted first, and those at both ends of cavities had the lowest temperatures and were melted last. 4.2. Impact of paraffin types

Fig. 6. Volumes and melting time of paraffin in Case1 to Case4.

paraffin that could be stuffed in Case1 was about 1.55  104 m3 which needed around 995 s to be melted completely. Case4 had the smallest volume of paraffin and the fastest melting rate with the most of thin sheets. The results suggest that with the precondition of enough paraffin to ensure that the LT is not reached, the higher melting rate of the paraffin is, the lower MCT is obtained. However, there is a tradeoff between the enhancement of the melting rate and the volume decrease of the paraffin limited by the confined cavities. In Fig. 7, some contour pictures about the temperatures and liquid fractions of Case0 and Case1 at s = 1500 s are shown. The middle part of the motors, both with the casings with or without paraffin, suffered the highest temperature in the whole working period because of the position of stator as the heat source. But the maximum temperature of the motor with Case1 was evidently decreased by 3 K compared with that of motor with Case0. On the bottom left of Fig. 7, the temperature distribution of paraffin in Case1 was characterized with a higher temperature at the middle position and lower values at the two ends. Correspondingly, the picture on the lower right presents the liquid fraction contour of

Fig. 8 depicts the effect of the paraffin types on the MCTs of Case4 with a continuous 270 W heating for 5000 s. Four kinds of paraffin with different melting temperatures listed in Table 2 were employed. With the same initial temperature, the start times of the four kinds of paraffin’s phase change processes were different for their distinct melting temperatures. All of the casings with paraffin performed better than Case0 without paraffin in respect of DWT prolonging, and a better temperature control performance was obtained when the paraffin with a higher melting temperature was utilized. As long as the phase change is efficient enough to ensure that the LT is not reached, the bigger temperature difference between the ambient air and the casing is, the more heat dissipation is obtained through the fins. In turn, the efficient temperature control time will last longer with a slower melting rate caused by a smaller heat flux into the paraffin. The results suggest that to get a better cooling effect of the motor with this paraffin-based casing the paraffin with a higher melting point below the LT should be chosen on the premise that an effective latent heat could be activated. 4.3. Thermal control performance under different working conditions 4.3.1. At different ambient temperatures Fig. 9 shows the transient MCTs of Case0 and Case4 with paraffin RT90 at various ambient temperatures. The heating process lasted for 2000 s which was long enough for a complete melting of the paraffin, and a peak temperature appeared at the end of this period. Then a natural cooling process of the motor was simulated during an interrupt time of 3000 s in which the paraffin could be re-solidified fully. The DWT of the motor decreased and the solidification time of the paraffin was extended when the surrounding temperature rose. But the prolonged rate of DWT with Case4 to that with Case0 increased with the rise of environmental temper-

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Fig. 7. Temperature and liquid fraction simulation results of Case0 and Case1 at s = 1500 s.

Fig. 8. Effect of paraffin types on maximum casing temperature of Case4.

ature (32.7%, 30.4% and 58.8% were obtained at the temperatures of 298.15 K, 323.15 K and 353.15 K respectively). Furthermore, the peak temperature of the motor was dropped by using the paraffin-based Case4 in each group of MCT lines. Fig. 10 compares the proportions of heat stored by paraffin and heat dissipation through the fins during the melting progress of paraffin. Evidently, the proportion of heat absorbed by the paraffin increased with the rise of environment temperature. These finds are understandable because the heat dissipated by the fins decreases for the reduced temperature difference between air and the casing, and more heat is stored in the paraffin. Thus to

Fig. 9. Transient maximum casing temperatures of Case0 and Case4 with RT90 for heating and cooling at various ambient temperatures.

keep a good thermal control effect, the mass of paraffin should be added according to the increased boundary temperature of the motor. 4.3.2. Under different heat load duty cycles Fig. 11 presents four groups of MCT curves of Case0 and Case4 with different paraffin types under various duty cycles. Under a duty cycle of 0.33 (D = 300 s:900 s, sload = 300 s and soff = 600 s), the MCTs of Case0 and Case4 with RT60 and RT90 respectively are drawn in the first group. The MCT line of Case4 with RT90

S. Wang et al. / Applied Thermal Engineering 109 (2016) 251–260

259

Fig. 10. Effect of different surrounding temperatures on heat storage and dissipation proportion with Case4.

Under the duty cycle of 0.67 (D = 600 s:900 s) in the third group, the highest temperatures of Case4 with RT90 in the third and the tenth cycles were about 7.82 K and 3.05 K cooler respectively than those of Case0. Finally, in comparison with the MCT of Case0 under the duty cycle of 0.83 (D = 750 s:900 s), the lower MCTs were appeared only in the second and third working cycles. The results suggest that the duty cycle of the motor is an important factor influencing the thermal control effect of the paraffinbased casing. When a higher duty cycle is applied, the motor implies a larger mean heat loss power leading to a higher temperature of the casing, and the paraffin with higher melting temperature should be applied. A long time effective cooling of the motor could be obtained as long as the interrupting time in each duty cycle is long enough for a completely re-solidification of the paraffin.

5. Conclusions This paper has proposed a temperature control method with a special closed motor casing packaged by paraffin-PCM for a PMSM involved in the actuator systems for aircraft. The impact of paraffin cavity configurations and the paraffin types on the transient cooling performance of the paraffin-based casings have been analyzed with the finite element method under different ambient temperatures and various heat load duty cycles. The main results were concluded as follows.

Fig. 11. Depiction of transient maximum casing temperatures of Case0 and Case4 with various duty cycles and different paraffin types.

paraffin was almost the same with that of Case0 and the latent heat of RT90 was not activated. However, compared with the MCTs of Case0, the peak temperature of Case4 in each cycle was much lower when RT60 was chosen as the PCM, but the valley value was relatively higher. Similarly, when the duty cycle was raised to 0.5 (D = 450 s:900 s) in the second group, the MCTs of Case4 with RT90 have little difference with those of Case0, while a lower peak value of MCT in each period (about 4.33 K cooler in the tenth cycle) was obtained when Case0 was replaced by Case4 with RT80.

(1) Compared with the conventional casing with fin for a selfcooling PMSM, the duration of working time of the motor could be extended by almost 32.7% with this new casing packaged by paraffin-PCM when a continuous 270 W heat power was generated inside the motor; and the peak temperature of the PMSM could be decreased by about 7.82 K when the RT90 paraffin was utilized in the casing under a duty cycle of 0.67. (2) To get a better cooling effect of the motor with this paraffinbased casing, the paraffin with a higher melting point below the heat-resistance temperature of the motor should be chosen on the premise that an effective latent heat could be activated, and the mass of paraffin should be added according to the increased boundary temperature of the motor. (3) The duty cycle of the motor is an important factor influencing the thermal control effect of the paraffin-based casing. A long time effective cooling of the motor could be obtained as long as the interrupting time in each duty cycle is long enough for a completely re-solidification of the paraffin. Overall, the cooling approach with the paraffin-PCM has superiority over the traditional way in achieving a better cooling effect

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with a simple structure for a PMSM. Furthermore, the improved heat managing ability can help to reduce the cooling surfaces and the size of motors involved in the actuator systems for aircraft. References [1] H.T. Qi, Y.L. Fu, X.Y. Qi, Y. Lang, Architecture optimization of more electric aircraft actuation system, Chin. J. Aeronaut. 24 (2011) 506–513. [2] W.P. Cao, B.C. Mecrow, G.J. Atkinson, J.W. Bennett, D.J. Atkinson, Overview of electric motor technologies used for more electric aircraft (MEA), IEEE Trans. Industr. Electron. 9 (59) (2012) 3523–3531. [3] R.J. Kang, Z.X. Jiao, S.P. Wang, L.S. Chen, Design and simulation of electrohydrostatic actuator with a built-in power regulator, Chin. J. Aeronaut. 22 (2009) 700–706. [4] J.W. Bennett, B.C. Mecrow, A.G. Jack, D.J. Atkinson, A prototype electrical actuator for aircraft flags, IEEE Trans. Ind. Appl. 46 (3) (2010) 915–921. [5] S. Morimoto, Y. Tong, Y. Takeda, T. Hirasa, Loss minimization control of permanent magnet synchronous motor drives, IEEE Trans. Ind. Appl. 14 (5) (1994) 511–517. [6] K. Yamazaki, Y. Seto, Iron loss analysis of interior permanent-magnet synchronous motors-variation of main loss factors due to driving condition, IEEE Trans. Ind. Appl. 42 (4) (2006) 1045–1052. [7] I. Moir, A. Seabridge, Aircraft Systems: Mechanical, Electrical, and Avionics Subsystems Integration, third ed., John Wiley & Sons Inc, Hoboken, 2008. [8] H. Guo, J.Q. Xu, X.L. Kuang, A novel tolerant permanent magnet synchronous motor with improved optimal torque control for aerospace application, Chin. J. Aeronaut. 28 (2) (2015) 535–544. [9] J.T. Shi, Z.Q. Zhu, et al., Comparative study of synchronous machines having permanent magnet in stator, Electr. Pow. Syst. Res. 133 (2016) 304–312. [10] K. Farsane, P. Desevaux, P.K. Panday, Experimental study of the cooling of a closed type electric motor, Appl. Therm. Eng. 20 (2000) 1321–1334. [11] C. Micallef, S.J. Pickering, K.A. Simmons, et al., Improved cooling in the end region of a strip-wound totally enclosed fan-cooled induction electric machine, IEEE Trans. Industr. Electron. 55 (10) (2008) 3517–3524. [12] J. Pyrhonen, P. Lindh, M. Polikarpova, et al., Heat-transfer improvements in an axial-flux permanent-magnet synchronous machine, Appl. Therm. Eng. 76 (2015) 245–251. [13] D.P. Kulkarni, G. Rupertus, E. Chen, Experimental investigation of contact resistance for water cooled jacket for electric motors and generators, IEEE Trans. Energy Convers. 27 (1) (2012) 204–210. [14] T. Davin, J. Pelle, S. Harmand, R. Yu, Experimental study of oil cooling systems for electric motors, Appl. Therm. Eng. 75 (2015) 1–13. [15] D.H. Lim, S.C. Kim, Thermal performance of oil spray cooling system for inwheel motor in electric vehicles, Appl. Therm. Eng. 63 (2014) 577–587. [16] F. Agyenim, N. Hewitt, P. Eaes, et al., A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS), Renew. Sustain. Energy Rev. 14 (2010) 615–628.

[17] Y.S. Li, L.J. Wang, J.F. Li, Paraffin Wax Produces Manuals, China Petrochemical Press, Beijing, 2009. [18] S. Sari-Bey, M. Fois, I. Krupa, et al., Thermal characterization of polymer matrix composites containing microencapsulated paraffin in solid or liquid state, Energy Convers. Manage. 78 (2014) 796–804. [19] W. Wu, G. Zhang, X. Ke, et al., Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management, Energy Convers. Manage. 101 (2015) 278–284. [20] Z.L. Ji, Y.H. Li, Y.Z. Li, Thermo-plastic finite element analysis for metal honeycomb structure, Therm. Sci. 17 (5) (2013) 1285–1291. [21] I. Krupa, Z. Nógellová, Z. Špitalsky´, et al., Phase change materials based on high-density polyethylene filled with microencapsulated paraffin wax, Energy Convers. Manage. 87 (2014) 400–409. [22] M.J. Allen, T.L. Bergman, A. Faghri, et al., Robust heat transfer enhancement during melting and solidification of a phase change material using a combined heat pipe-metal foam or foil configuration, J. Heat Transfer 137 (2015) 1–15. [23] J. Turnpenny, D. Etheridge, D. Reay, Novel ventilation cooling system for reducing air conditioning in buildings. Part I: Testing and theoretical modelling, Appl. Therm. Eng. 20 (2000) 1019–1037. [24] J. Turnpenny, D. Etheridge, D. Reay, Novel ventilation cooling system for reducing air conditioning in buildings. Part II: Testing and prototype, Appl. Therm. Eng. 21 (2001) 1203–1217. [25] N.R. Jankowski, F.P. Mccluskey, A review of phase change material for vehicle component thermal buffering, Appl. Energy 113 (2014) 1525–1561. [26] F.L. Tan, S.C. Fok, Cooling of helmet with phase change material, Appl. Therm. Eng. 26 (2006) 2067–2072. [27] F.L. Tan, C.P. Tso, Cooling of mobile electronic devices using phase change materials, Appl. Therm. Eng. 24 (2004) 159–169. [28] R. Kandasamy, X.Q. Wang, A.S. Mujumdar, Transient cooling of electronics using phase change material (PCM)-based heat sinks, Appl. Therm. Eng. 28 (2008) 1047–1057. [29] T.J. Lu, Thermal management of high power electronics with phase change cooling, Int. J. Heat Mass Transfer 43 (2000) 2245–2256. [30] J. Bellettre, V. Sartre, F. Biais, A. Lallemand, Transient state study of electric motor heating and phase change solid-liquid cooling, Appl. Therm. Eng. 17 (1) (1997) 17–31. [31] S.N. Wang, Y.Z. Li, Y. Liu, et al., Temperature control of permanent-magnet synchronous motor using phase change material, in: IEEE/ASME International Conference on Advanced Intelligent Mechatronics, July 7–11, 2015, Busan, Korea. [32] S.D. Umans, Fitzgerald & Kingsley’s Electric Machinery, seventh ed., McGrawHill Education, New York, 2014. [33] F.P. Incropera, D.P. Dewitt, T.L. Bergman, et al., translated by X.S. Ge and H. Ye. Fundamentals of Heat and Mass Transfer (sixth ed.), Chemical Industry Press, Beijing, 2007. [34] Z.Q. Hou, J.G. Hu, Fundamental and Application of Spacecraft Thermal Control Technology, China Science and Technology Press, Beijing, 2007.