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Numerical analysis of temperature distribution of motor-refrigerant in a R32 rotary compressor
Applied Thermal Engineering 95 (2016) 365–373
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Numerical analysis of temperature distribution of motor-refrigerant in a R32 rotary compressor Jianhua Wu a,*, Jiehao Hu a, Ang Chen a, Peipei Mei b, Xingbiao Zhou b, Zhenhua Chen b a b
School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Guangdong Meizhi Compressor Co. Ltd, Guangdong 528333, China
H I G H L I G H T S
• • • • •
The rotary compressor is divided into upper part and the lower part to analysis. Motor-refrigerant in a R32 rotary compressor is modeled for CFD simulation. The clearance between motor rotor and stator are taken into account. 3D temperature field and flow field of motor-refrigerant in a R32 rotary compressor are obtained. Some methods need to be adopted to reduce the high temperature of stator winding under some conditions.
A R T I C L E
I N F O
Article history: Received 26 May 2015 Accepted 12 November 2015 Available online 2 December 2015 Keywords: Rotary compressor Motor R32 CFD Temperature distribution Fluid–solid coupling
A B S T R A C T
All the time the reliability of motor is one of the important factors affecting the lifespan of compressor. It is particularly severe for motor in a R32 hermetic rotary compressor, which is cooled by discharge R32 at high pressure and temperature. This paper presents a method to obtain the 3-D temperature distribution of motor-refrigerant in a R32 hermetic rotary compressor. Fluid–solid coupling analysis method was used to simulate the heat transfer between refrigerant and motor. The clearance between motor rotor and stator, the influence of rotating balancer attached to rotor on the fluid field and the impact of heat insulation material on heat transfer are taken into account. The simulation results fit the experimental data well. The analytical conclusions would be useful for further understanding of heat transfer in motor, and also provide a guideline of designing and improving compressor for manufacturers. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction In order to avoid the further expansion of Antarctic ozone hole and decrease greenhouse effect, people are eager to seek new refrigerants with both low Ozone Depression Potential (ODP) and low Global Warming Potential (GWP). In recent years, total equivalent warming impact (TEWI) has been adopted to evaluate contributions of refrigerants to global warming for considering both the direct emissions and indirect emissions of refrigerants. R32, as one of alternative refrigerants, belongs to hydrofluorocarbons, and has similar thermal physical properties with R410A, while its GWP is 675, only 1/3 of R410A. Based on above advantages, R32 has been used in room air conditioners as a more sustainable choice than R410A by some air conditioning manufacturers [1]. However, there is an unnegligible problem for R32: a rather higher discharge temperature than those of R22 and R410A.
* Corresponding author. Tel.: +86 029 82663786. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.applthermaleng.2015.11.036 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
At present, room air conditioners generally use hermetic rotary compressors due to its good reliability, high efficiency, and compact structure. The hermetic rotary compressors adopt the high back pressure structure, namely, inside the shell is full of discharge gas at high pressure and temperature, using discharge gas to cool the motor and other components. This structure can greatly decrease the suction gas heating loss comparing the low back pressure structure; however, it brings the heat dissipation problem, especially for the R32 hermetic compressor. Motor burnout is a common failure of compressors, and stator winding is the place where insulation failure most possibly happens, so it is necessary to investigate the temperature of the motor in R32 hermetic rotary compressors with high back pressure structure. In the past, researchers generally adopt lumped parameter method to study the temperature distribution of the compressor, namely, dividing the compressor into a series of lumped elements, each of which is indicated by a thermal node, then conducting a iterative calculation with programs. Padhy and Dwivedi [2] divided a rotary compressor into 22 elements, Liu [3] divided the rotary compressor into 8 elements, they all treat the stator as an element. Chen
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et al. [4] also used lumped parameter method to study the temperature of the motor in semi-hermetic screw compressor, while they divide the stator of the motor into several parts, thus, the results are closer to the actual temperature distribution in motor stator. However, for lumped parameter method, the mathematic formulas used to calculate heat transfer between components of compressor are complicated, and the temperature distribution in an element cannot be obtained. With the development of computer, computational fluid dynamics (CFD) technology is used to calculate the flow or temperature field in reciprocal, scroll, screw compressors [5–10], while the temperature distribution of rotary compressor has not yet been investigated using CFD. In this paper, a CFD method to obtain the 3-D temperature distribution of motor-refrigerant in a R32 hermetic rotary compressor is presented. The clearance between motor rotor and stator, the influence of rotating balancer attached to rotor on the fluid field and the impact of heat insulation material on heat transfer are taken into account. Meanwhile, the temperatures of stator winding, stator iron, and refrigerant are measured experimentally to validate the model. The simulated results would be useful for further understanding of heat transfer in motor, and also can be a reference of design and improvement for compressor manufacturers. 2. Modeling and meshing Fig. 1 illustrates the structure of a hermetic rotary compressor. It is mainly composed of several components: a motor, a crankshaft, a muffler, a main bearing, a cylinder, a roller, a sub bearing, an accumulator, a vane and a shell. The motor in the rotary compressor comprises a stator winding, an iron core and a rotor. The fluids in the shell of the compressor contain refrigerant and oil. The mixed-up liquid is in contact with components inside the compressor. For the ease of calculation, this paper divides the compressor into upper part and lower part. The upper part begins from the outlet of the muffler and ends at the inlet of the discharge pipe, including stator winding, iron core, rotor, shell, refrigerant among the former components, and air outside the shell in the corresponding area. The lower part comprises pump, refrigerant in the cylinder, oil sump, and air outside the shell in the corresponding height. Considering the length of the paper, we only analyze the upper part. Losses inside the three-phase asynchronous induction motor can
be divided into iron losses, ohmic losses and stray load losses by the induced mechanism [4]. The motor losses mostly turn into heat. During operation of the compressor, the motor generates heat; the refrigerant at high temperature and high pressure firstly discharges from the muffler, then flows through the stator cooling ducts, the gap between stator and rotor, meanwhile cools the motor, and finally discharges from the compressor. On the other hand, the heat of the motor and refrigerant will transfer to ambient air in the form of convection and radiation heat transfer through the shell. At last, the heat transfer among motor, shell, and refrigerant reach a balancing point and their temperature no longer change. 2.1. Physical model The computational domain of the fluid–solid coupling model is depicted in detail in Fig. 2. This model comprises fluid domain, i.e. refrigerant R32; solid domain, i.e. stator winding and stator core. Three bodies are modeled respectively, and then assembled together. The interface of any two bodies is acted as the public face, and the fluid domain and solid domain are coupling calculated. 2.1.1. Refrigerant R32 The refrigerant gas occupies the rest of space except the solid components inside the model, i.e. the upper and low cavity of motor, the gap between rotor and stator, cooling ducts between iron core and internal face of shell. The refrigerant is assumed as an incompressible fluid, because the velocity of refrigerant in compressor is low, Mach number is far less than 0.3. 2.1.2. Iron core and stator winding This stator iron core has 24 teeth, and is compacted by a number of 0.5 mm thick silicon steel plates. Thermal contact resistance between two adjacent silicon steel sheets is ignored, as silicon steel sheets are tightly bound and the surface of silicon steel sheet is flat and smooth. The actual impact of thermal contact resistances on axial thermal resistance of stator iron core will be measured in later work. Stator winding comprises upper end-winding, lower endwinding, and middle winding located in the slot of stator iron core. The insulation varnish of stator winding, insulation paper in stator slot, and gas in stator slot are equivalent to insulators between slot winding and stator iron. Insulators are located in white zones of stator winding model in Fig. 2. 2.1.3. Rotor For the purpose of reducing the computational domain, the model doesn’t contain the rotor. Instead, only the outer profile face of the rotor is reserved, i.e. outer circumferential surface, top and bottom surface, outface of upper and lower balancer. 2.2. Meshing FEM pre-processing software gambit is used to discrete the computational domain, and the discrete result is shown in Fig. 3. It comprises approximately 5,700,000 tetrahedral unstructured grids. Grids of the places with a large gradient should be refined, such as stator cooling ducts, gap between stator and rotor, etc. Meanwhile, grids of the places with a small gradient should be meshed sparsely for the benefit of higher computational efficiency. The Grid independence test is shown in Table 1. 2.3. Boundary conditions
Fig. 1. Sectional view of a hermetic rotary compressor.
2.3.1. Inlet of model The compressed refrigerant at high temperature discharges from the outlet of muffler, then enter into the cavity underneath the motor.
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Fig. 2. Physical model of motor fluid-solid coupling.
temperature is selected as the temperature of inlet boundary condition.
2.3.2. Outlet of model The refrigerant gas takes away the heat generated by the motor, then discharges from the outlet of model, enters into condenser and releases condensation heat at last. The physical information about outlet of model is unknown in advance, so the outlet of model is set as outflow condition.
Fig. 3. Schematic of computational grid.
The inlet of this model is set as constant velocity-inlet condition with flow direction normal to the cross sectional area (50.3 mm2) of the inlet. The flow rate in ARI condition is 51.3 kg/h, resulting in 4.2 m/s for the refrigerant density at that condition. The discharge Table 1 Grid independence test. Number of element
Temperature of outlet (K)
Velocity of outlet (m/s)
2,126,432 5,726,584 7,058,650
391.36 391.41 391.42
2.67 2.80 2.77
2.3.3. Thermal sources term Squirrel cage of three-phase asynchronous induction motor is selected as the power source of this rotary compressor. During the operation of the motor, motor losses turn into heat, which leads to output power is less than input power. Motor losses, mostly appearing in the form of heat energy, are internal factors leading to motor heating. Motor losses comprise mechanical and electrical losses. Electrical losses can be divided into copper losses, iron losses, rotor aluminum losses and stray losses. Frequency of induction electric field in rotor is very low, only 1~3 Hz, while core loss is proportion to frequency, so the rotor core losses can be ignored. Copper losses Pcu can be calculated as
Pcu = 3I 2R
(1)
Where I, R is phase current and phase resistance of stator winding, respectively. Rotor aluminum losses Pal is obtained by using electromagnetic field analysis software Ansoft 14.0. The stator iron and rotor were not simplified, just stator winding was treated as a whole. The grid size range is between 0.5 mm and 3 mm. Stator iron losses equal motor total losses minus copper losses and aluminum losses. According to Pcu, Piron and the two part volume, their heat production rates per unit volume are easy to acquire, before they are added
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in the cell zone conditions as source terms. The aluminum losses are added in the outer circumferential surface of the rotor. 2.3.4. Coupled condition Contact face of any two bodies belongs to coupling surface, through which the two bodies reciprocally convey physical information. Insulators between stator winding and stator iron core are realized by the shell conduction function in Fluent. Shell conduction can be used to model one or more layers of wall cells without the need to mesh the wall thickness in a preprocessor [11]. 2.3.5. Wall condition During the steady operation of rotary compressor, rotor along with upper and lower balancer rotates at certain rotational speed n. Consequently, outer surface of rotor and upper and lower balancer should be set as rotary wall condition. Rotational speed n of three-phase asynchronous induction motor can be represented as
n=
60[s min]f p (1 − s )
(2)
Where Ck and Gb represent the generation of turbulence kinetic energy due to the mean velocity and buoyancy, respectively, YM denotes the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, C1ε, C2ε, C3ε, σκ and σε are default values from ANSYS, Sκ and Sε are user-defined source terms. 3. Experimental verification In order to verify accuracy of this model, temperatures at certain positions inside a rotary compressor under different conditions are measured. The measured rotary compressor is shown in Fig. 4. During measurement, the measured compressor using R32 as refrigerant is placed in the Calorie Tester. The test conditions and main parameters of the measured compressor are shown in Tables 2 and 3. Of the several conditions in Table 3, the high efficiency condition denotes the condition was designed to match the air conditioner with COP of 3.4–3.6. Since the size of rotary compressor used in room air conditioner is small and compact, the number of drilling holes on the
where f is frequency of A.C, p is pole pairs, s represents slip ratio. The temperature of the shell is higher than that of surrounding air, so the heat is transferred from shell to surrounding air. In the case of natural convection, radiation heat transfer has the same order of magnitude as convection heat transfer, so radiation heat transfer cannot be ignored. This boundary condition can be presented in the form
−λ
∂T ∂n
wall
= h ( Tw − Tf ) + εσ ( Tw4 − T∞4 )
(3)
Where λ is the thermal conductivity of shell, n is exterior normal direction of the boundary, h is the convective heat transfer coefficient, Tw is temperature of compressor shell, Tf is the temperature of the surrounding air, ε is emissivity of solid surface, σ is Stefan– Boltzmann constant, and T∞ is the temperature of surrounding wall. 2.4. Fundamental equations The heat conduction inside the solid in a three-dimensional Cartesian coordinate system should obey the following law:
∂ ∂x
⎛ ∂T ⎞ ∂ ⎜⎝ λ x ⎟+ ∂x ⎠ ∂y
∂T ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ ⎜⎝ λ y ∂y ⎟⎠ + ∂z ⎜⎝ λz ∂z ⎟⎠ + q = ρc ∂t
(4)
Where T is temperature, λx, λy, λz is thermal conductivity of solid in x, y, z direction, respectively, t denotes time, q is the heat generation rate per unit volume. Due to the steady operation state of the compressor, the right hand item should be zero. During the actual operation of compressor, flow of refrigerant in the cavity of compressor is turbulent flow, so the standard k-ε turbulent model provided by ANSYS Fluent is adopted to model the flow of refrigerant. The turbulence kinetic energy k, and its rate of dissipation ε, can be calculated by the following transport equations:
∂ ∂ ⎡⎛ μt ⎞ ∂κ ⎤ ∂ + Gκ + G b − ρε − Y M + S κ (ρκ ) + (ρκui ) = ⎜ μ + ⎟⎠ ∂x i ∂x j ⎢⎣⎝ σ κ ∂x j ⎥⎦ ∂t ∂ ∂ ⎡⎛ μt ⎞ ∂ε ⎤ ∂ (ρε ) + (ρεui ) = ⎜ μ + ⎟⎠ ∂x i ∂x j ⎢⎣⎝ σ ε ∂x j ⎥⎦ ∂t ε ε2 + C 1ε (Gκ + C 3εG b ) − C 2ε ρ + S ε κ κ
(5)
(6)
Fig. 4. View of the measured rotary compressor. (a) Iron core. (b) Sectional view of the compressor.
Table 2 Main parameters of the experimental rotary compressor. Parameter
Value
Rated frequency (Hz) Input power (W) Refrigerating capacity (W) Height of compressor (mm) Inner radius of shell (mm) Outer radius of shell (mm) Height of stator winding (mm) Outer radius of stator winding (mm) Height of stator iron core (mm)
50 1173 + /−3‰ 3415.6 268.0 110.4 117.4 150.0 96.0 90.0
Note: The refrigerating capacity and input power of compressor are measured in ARI condition [12].
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Table 3 Test conditions. Item
High eff.
High eff. I
High eff. II
ARI
ASHRAE/T1 [13]
Condensing Temp. (°C) Evaporating Temp. (°C) Suction Temp. (°C) Subcooling Temp. (°C) Ambient Temp. (°C)
46.0 10.0 18.0 41.0 35.0
46.0 10.0 24.0 41.0 35.0
46.0 10.0 30.0 41.0 35.0
54.4 7.2 18.3 46.1 35.0
54.4 7.2 35 46.1 35.0
Note: The measurement accuracy is +/−0.3 °C.
Table 4 The measuring-point items. Serial number
Item
1 2 3 4 5 6
Gas above the stator winding Stator winding Tooth of the upper stator iron Middle of the upper stator iron Tooth of the lower stator iron Gas under the stator winding
shell is limited. Furthermore, it is not realistic to install too many measurement components inside the shell. Drilling a hole with large radius on the shell may damage the strength of compressor shell, so a corresponding nut is welded on the outer face of shell; the sheathed thermocouple is then screwed onto the nut. In order to measure the temperature of refrigerant gas above the stator winding, a sheathed thermocouple with thread (ST, STTT-T-ATB2C15D1M5F2L1PIT5W0S0) was used. A similar procedure is done to measure the temperature of refrigerant gas under the stator winding. Stator winding is the place where it generates the most heat, so a T-type thermocouple is inserted in the stator winding to measure the temperature of stator winding. Furthermore, to measure the temperature of iron core, two T-type thermocouples are inserted in the tooth and middle of silicon steel plate in the upper portion of iron core, as is shown in Fig. 5a, while a T-type thermocouple is inserted in the middle of silicon steel plate in the lower portion of iron core. The measurement accuracies of sheathed thermocouple (STTT-T), T-type thermocouple are +/−0.5 °C and +/−0.3 °C, respectively. The measuring-point items are listed in Table 4. After thermocouples are installed in measuring position, these thermocouple wires should be taken out of the compressor through copper pipes welded on the shell, and the copper pipes should be sealed by using glue. During the experiment, the measured compressor is in the condition of natural convection. The temperatures of measured points are obtained after the stable operation of the compressor. Fig. 6 shows the comparisons between simulated results and experimental data under different conditions. It can be seen that the simulated results fits the experimental data well. Furthermore, the temperature of stator winding is higher than that of other places, especially the temperature of stator winding under the ASHRAE/T1 condition, which is 8.3 °C higher than the maximum acceptable temperature of the most widely used B-class motor insulation class, so some methods to decrease the compressor discharge temperature must be adopted, such as wet suction by adjusting the expansion valve, and the usage of lubricant oils specifically designed for R32 [14]. 4. Results and discussion Fig. 7 illustrates simulated results of temperature distribution of stator winding, stator core, refrigerant gas under ARI condition.
Iron core
Sectional view of the compressor Fig. 5. Arrangement of measuring points. (a) High eff. condition. (b) High eff. I condition. (c) High eff. II condition. (d) ARI condition. (e) ASHRAE/T1 condition
Figs. 8–10 show simulated results of temperature distribution of motor-refrigerant in y = 0 mm, A-A, z = 100.0 mm plane, respectively. During the stable operation of compressor, the rotor with the upper and lower balancer rotates at rotational speed n. Affected by this, the refrigerant gas in upper and lower cavity of compressor rotates, and adequately mixes. Consequently, the temperature of refrigerant is relatively uniform. The highest temperature of outer face in motor-refrigerant model appears in the place contact with stator iron. This is understandable because the thermal conductivity of solid stator iron is higher than that of refrigerant gas. Hence, the heat transferred from stator iron to surroundings due to heat conduction is larger than the heat transferred from the refrigerant gas to surroundings due to heat convection.
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High eff. condition
High eff. I condition
ARI condition
High eff. II condition
ASHRAE/T1 condition Fig. 6. Comparisons between simulated results and experimental data under 5 conditions. Note: the bars indicate zero order (only measurement apparatus) uncertainty
Figs. 8 and 9 show simulated results of temperature distribution of motor-refrigerant in y = 0 mm, A-A plane, respectively. The total temperature variations of stator winding are small, less than 2.5 °C. Furthermore, in general, the temperature of stator winding in the same height is about 1.3 °C higher than that of stator iron. This is mainly because heat per unit volume in stator winding is higher than that in stator iron. Furthermore, the heat of stator winding cannot sufficiently transfer to stator iron due to the existence of the insulator, so it appears apparent temperature boundary, as shown in Fig. 10.
The temperature of stator iron decreases with the increase of the radius in a horizontal section, mainly because the outer edge of stator iron is in contact with the refrigerant gas and shell with colder temperature. Moreover, affected by cooling passage arrangement, the temperature of stator iron in the same horizontal plane is not identical. Due to the small clearance between stator and rotor, the refrigerant flow rate is low. Further, the refrigerant gas here absorbs the heat caused by the rotor aluminum losses, stator iron losses, and copper losses, hence, its temperature rise is relatively high, up to about 8 °C. While the flow rate of the refrigerant gas in stator cooling
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371
Fig. 7. Simulated results of temperature distribution of motor-refrigerant.
ducts is high, and the refrigerant gas at high temperature dissipates heat to surroundings air through shell, so its temperature rise is low. The two gas flows mix in the motor upper cavity, and then discharge from discharge pipe.
When the compressor reaches steady state, heat transfer among refrigerant, motor and ambient air reaches a balance and the temperature inside the rotary compressor won’t change. The heat balance for the model can be presented in the following form:
Fig. 8. Simulated results of temperature distribution in y = 0 mm plane.
Fig. 9. Simulated results of temperature distribution in A-A plane.
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temperature rises as suction temperature rises, and shell temperature and natural convective heat transfer coefficient will increase, accordingly, so natural convective heat transfer is bigger. The heat generated by motor under ASHRAE/T1 condition is closer to that under ARI condition, while the heat dissipation is bigger than the latter, so the heat absorbed by refrigerant is less. Influenced by the rotation of rotor, the refrigerant gas in the annular gap between rotor and stator flows upward spirally. Under ARI condition, the mass flow rate of refrigerant gas in the annular gap between rotor and stator is 0.00473 kg/s; on the other hand, the refrigerant gas in the cooling ducts between iron core and shell flows upward, the mass flow rate in the cooling ducts between iron core and shell is 0.00937 kg/s.
5. Conclusions
Fig. 10. Simulated results of temperature distribution in z = 100.0 mm plane.
Q loss = Q refri + Q amb
(7)
Q amb = Q conv + Q radi
(8)
where Qloss represents the heat generated by motor, Qrefri is the heat absorbed by refrigerant, Qamb is the heat transferred between the shell and ambient air, including and heat transfer by natural convection Qconv and heat transfer by radiation Qradi. Qrefri can be calculated by the following equation:
( Toutlet − Tinlet ) Q refri = mC
(9)
where m is the mass flow rate of refrigerant, C represents specific heat of refrigerant, and Tinlet, Toutlet is the temperature of inlet and outlet of refrigerant, respectively. Qrefri, Qconv and Qradi under 5 conditions are shown in Fig. 11. The ratio of radiation and natural convective heat transfer ranges between 1.1 and 1.4, so the radiation heat transfer cannot be neglected when calculating the heat dissipation of shell under natural-convection environment. Comparisons of the former 3 conditions and the latter two conditions show that the convection heat transfer increases with an increase in suction temperature. This is because the discharge
In this study, the temperature distribution for motor-refrigerant in R32 hermetic rotary compressor is obtained by using fluid– solid coupling method and computational fluid dynamics technology. Based on the simulated results presented in this paper, the following conclusions can be drawn: 1. The temperature of stator winding under the ASHRAE/T1 condition is 8.3 °C higher than the maximum acceptable temperature of the most widely used B-class motor insulation class, so some methods to decrease the compressor discharge temperature must be adopted. 2. The simulated results fit the experimental data well. Under these test conditions, the maximum error is less than 5 °C, and it appears in tooth of the lower stator iron under ARI condition. 3. The total temperature variations of stator winding within the scope of their height are small, in ARI condition, less than 2.5 °C. It indicates that a lumped method is accurate enough for the temperature of the stator winding. 4. It is remarkable that the temperature rise of the refrigerant passing though between stator and rotor is, with up to 8 °C, much higher than for refrigerant that passes through the cooling ducts (up to 1.7 °C). 5. The ratio of radiation and natural convective heat transfer ranges between 1.1 and 1.4 under 5 conditions, so the radiation heat transfer cannot be neglected when calculating the heat dissipation of shell under natural-convection environment.
Fig. 11. Comparison of heat transfer rate under 5 conditions.
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Nomenclature [2]
c f h I n p R s t T λ ε σ ρ μ m
specific heat capacity (J/ (kg•K)) AC frequency (Hz) convective heat transfer coefficient (W/ (m2•K)) phase current (A) rotational speed (r/min) pole pairs (-) phase resistance (Ω) slip ratio (-) time (s) temperature (°C) thermal conductivity (W/ (m2•K)) emissivity (-) Stefan–Boltzmann constant (W/ (m2•K4)) density (kg/m3) dynamic viscosity (Pa•s) mass flow rate (kg/s)
Subscripts w wall surface f fluid ∞ infinity x x axis y y axis z z axis References [1] S. Taira, A. Nakai, R. Yajima, 2010. Evaluation and Practical Use of Equipment Using New Refrigerant as an Urgent Countermeasure against Global Warming.In:
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Numerical analysis of temperature distribution of motor-refrigerant in a R32 rotary compressor Jianhua Wu a,*, Jiehao Hu a, Ang Chen a, Peipei Mei b, Xingbiao Zhou b, Zhenhua Chen b a b
School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Guangdong Meizhi Compressor Co. Ltd, Guangdong 528333, China
H I G H L I G H T S
• • • • •
The rotary compressor is divided into upper part and the lower part to analysis. Motor-refrigerant in a R32 rotary compressor is modeled for CFD simulation. The clearance between motor rotor and stator are taken into account. 3D temperature field and flow field of motor-refrigerant in a R32 rotary compressor are obtained. Some methods need to be adopted to reduce the high temperature of stator winding under some conditions.
A R T I C L E
I N F O
Article history: Received 26 May 2015 Accepted 12 November 2015 Available online 2 December 2015 Keywords: Rotary compressor Motor R32 CFD Temperature distribution Fluid–solid coupling
A B S T R A C T
All the time the reliability of motor is one of the important factors affecting the lifespan of compressor. It is particularly severe for motor in a R32 hermetic rotary compressor, which is cooled by discharge R32 at high pressure and temperature. This paper presents a method to obtain the 3-D temperature distribution of motor-refrigerant in a R32 hermetic rotary compressor. Fluid–solid coupling analysis method was used to simulate the heat transfer between refrigerant and motor. The clearance between motor rotor and stator, the influence of rotating balancer attached to rotor on the fluid field and the impact of heat insulation material on heat transfer are taken into account. The simulation results fit the experimental data well. The analytical conclusions would be useful for further understanding of heat transfer in motor, and also provide a guideline of designing and improving compressor for manufacturers. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction In order to avoid the further expansion of Antarctic ozone hole and decrease greenhouse effect, people are eager to seek new refrigerants with both low Ozone Depression Potential (ODP) and low Global Warming Potential (GWP). In recent years, total equivalent warming impact (TEWI) has been adopted to evaluate contributions of refrigerants to global warming for considering both the direct emissions and indirect emissions of refrigerants. R32, as one of alternative refrigerants, belongs to hydrofluorocarbons, and has similar thermal physical properties with R410A, while its GWP is 675, only 1/3 of R410A. Based on above advantages, R32 has been used in room air conditioners as a more sustainable choice than R410A by some air conditioning manufacturers [1]. However, there is an unnegligible problem for R32: a rather higher discharge temperature than those of R22 and R410A.
* Corresponding author. Tel.: +86 029 82663786. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.applthermaleng.2015.11.036 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
At present, room air conditioners generally use hermetic rotary compressors due to its good reliability, high efficiency, and compact structure. The hermetic rotary compressors adopt the high back pressure structure, namely, inside the shell is full of discharge gas at high pressure and temperature, using discharge gas to cool the motor and other components. This structure can greatly decrease the suction gas heating loss comparing the low back pressure structure; however, it brings the heat dissipation problem, especially for the R32 hermetic compressor. Motor burnout is a common failure of compressors, and stator winding is the place where insulation failure most possibly happens, so it is necessary to investigate the temperature of the motor in R32 hermetic rotary compressors with high back pressure structure. In the past, researchers generally adopt lumped parameter method to study the temperature distribution of the compressor, namely, dividing the compressor into a series of lumped elements, each of which is indicated by a thermal node, then conducting a iterative calculation with programs. Padhy and Dwivedi [2] divided a rotary compressor into 22 elements, Liu [3] divided the rotary compressor into 8 elements, they all treat the stator as an element. Chen
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et al. [4] also used lumped parameter method to study the temperature of the motor in semi-hermetic screw compressor, while they divide the stator of the motor into several parts, thus, the results are closer to the actual temperature distribution in motor stator. However, for lumped parameter method, the mathematic formulas used to calculate heat transfer between components of compressor are complicated, and the temperature distribution in an element cannot be obtained. With the development of computer, computational fluid dynamics (CFD) technology is used to calculate the flow or temperature field in reciprocal, scroll, screw compressors [5–10], while the temperature distribution of rotary compressor has not yet been investigated using CFD. In this paper, a CFD method to obtain the 3-D temperature distribution of motor-refrigerant in a R32 hermetic rotary compressor is presented. The clearance between motor rotor and stator, the influence of rotating balancer attached to rotor on the fluid field and the impact of heat insulation material on heat transfer are taken into account. Meanwhile, the temperatures of stator winding, stator iron, and refrigerant are measured experimentally to validate the model. The simulated results would be useful for further understanding of heat transfer in motor, and also can be a reference of design and improvement for compressor manufacturers. 2. Modeling and meshing Fig. 1 illustrates the structure of a hermetic rotary compressor. It is mainly composed of several components: a motor, a crankshaft, a muffler, a main bearing, a cylinder, a roller, a sub bearing, an accumulator, a vane and a shell. The motor in the rotary compressor comprises a stator winding, an iron core and a rotor. The fluids in the shell of the compressor contain refrigerant and oil. The mixed-up liquid is in contact with components inside the compressor. For the ease of calculation, this paper divides the compressor into upper part and lower part. The upper part begins from the outlet of the muffler and ends at the inlet of the discharge pipe, including stator winding, iron core, rotor, shell, refrigerant among the former components, and air outside the shell in the corresponding area. The lower part comprises pump, refrigerant in the cylinder, oil sump, and air outside the shell in the corresponding height. Considering the length of the paper, we only analyze the upper part. Losses inside the three-phase asynchronous induction motor can
be divided into iron losses, ohmic losses and stray load losses by the induced mechanism [4]. The motor losses mostly turn into heat. During operation of the compressor, the motor generates heat; the refrigerant at high temperature and high pressure firstly discharges from the muffler, then flows through the stator cooling ducts, the gap between stator and rotor, meanwhile cools the motor, and finally discharges from the compressor. On the other hand, the heat of the motor and refrigerant will transfer to ambient air in the form of convection and radiation heat transfer through the shell. At last, the heat transfer among motor, shell, and refrigerant reach a balancing point and their temperature no longer change. 2.1. Physical model The computational domain of the fluid–solid coupling model is depicted in detail in Fig. 2. This model comprises fluid domain, i.e. refrigerant R32; solid domain, i.e. stator winding and stator core. Three bodies are modeled respectively, and then assembled together. The interface of any two bodies is acted as the public face, and the fluid domain and solid domain are coupling calculated. 2.1.1. Refrigerant R32 The refrigerant gas occupies the rest of space except the solid components inside the model, i.e. the upper and low cavity of motor, the gap between rotor and stator, cooling ducts between iron core and internal face of shell. The refrigerant is assumed as an incompressible fluid, because the velocity of refrigerant in compressor is low, Mach number is far less than 0.3. 2.1.2. Iron core and stator winding This stator iron core has 24 teeth, and is compacted by a number of 0.5 mm thick silicon steel plates. Thermal contact resistance between two adjacent silicon steel sheets is ignored, as silicon steel sheets are tightly bound and the surface of silicon steel sheet is flat and smooth. The actual impact of thermal contact resistances on axial thermal resistance of stator iron core will be measured in later work. Stator winding comprises upper end-winding, lower endwinding, and middle winding located in the slot of stator iron core. The insulation varnish of stator winding, insulation paper in stator slot, and gas in stator slot are equivalent to insulators between slot winding and stator iron. Insulators are located in white zones of stator winding model in Fig. 2. 2.1.3. Rotor For the purpose of reducing the computational domain, the model doesn’t contain the rotor. Instead, only the outer profile face of the rotor is reserved, i.e. outer circumferential surface, top and bottom surface, outface of upper and lower balancer. 2.2. Meshing FEM pre-processing software gambit is used to discrete the computational domain, and the discrete result is shown in Fig. 3. It comprises approximately 5,700,000 tetrahedral unstructured grids. Grids of the places with a large gradient should be refined, such as stator cooling ducts, gap between stator and rotor, etc. Meanwhile, grids of the places with a small gradient should be meshed sparsely for the benefit of higher computational efficiency. The Grid independence test is shown in Table 1. 2.3. Boundary conditions
Fig. 1. Sectional view of a hermetic rotary compressor.
2.3.1. Inlet of model The compressed refrigerant at high temperature discharges from the outlet of muffler, then enter into the cavity underneath the motor.
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Fig. 2. Physical model of motor fluid-solid coupling.
temperature is selected as the temperature of inlet boundary condition.
2.3.2. Outlet of model The refrigerant gas takes away the heat generated by the motor, then discharges from the outlet of model, enters into condenser and releases condensation heat at last. The physical information about outlet of model is unknown in advance, so the outlet of model is set as outflow condition.
Fig. 3. Schematic of computational grid.
The inlet of this model is set as constant velocity-inlet condition with flow direction normal to the cross sectional area (50.3 mm2) of the inlet. The flow rate in ARI condition is 51.3 kg/h, resulting in 4.2 m/s for the refrigerant density at that condition. The discharge Table 1 Grid independence test. Number of element
Temperature of outlet (K)
Velocity of outlet (m/s)
2,126,432 5,726,584 7,058,650
391.36 391.41 391.42
2.67 2.80 2.77
2.3.3. Thermal sources term Squirrel cage of three-phase asynchronous induction motor is selected as the power source of this rotary compressor. During the operation of the motor, motor losses turn into heat, which leads to output power is less than input power. Motor losses, mostly appearing in the form of heat energy, are internal factors leading to motor heating. Motor losses comprise mechanical and electrical losses. Electrical losses can be divided into copper losses, iron losses, rotor aluminum losses and stray losses. Frequency of induction electric field in rotor is very low, only 1~3 Hz, while core loss is proportion to frequency, so the rotor core losses can be ignored. Copper losses Pcu can be calculated as
Pcu = 3I 2R
(1)
Where I, R is phase current and phase resistance of stator winding, respectively. Rotor aluminum losses Pal is obtained by using electromagnetic field analysis software Ansoft 14.0. The stator iron and rotor were not simplified, just stator winding was treated as a whole. The grid size range is between 0.5 mm and 3 mm. Stator iron losses equal motor total losses minus copper losses and aluminum losses. According to Pcu, Piron and the two part volume, their heat production rates per unit volume are easy to acquire, before they are added
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in the cell zone conditions as source terms. The aluminum losses are added in the outer circumferential surface of the rotor. 2.3.4. Coupled condition Contact face of any two bodies belongs to coupling surface, through which the two bodies reciprocally convey physical information. Insulators between stator winding and stator iron core are realized by the shell conduction function in Fluent. Shell conduction can be used to model one or more layers of wall cells without the need to mesh the wall thickness in a preprocessor [11]. 2.3.5. Wall condition During the steady operation of rotary compressor, rotor along with upper and lower balancer rotates at certain rotational speed n. Consequently, outer surface of rotor and upper and lower balancer should be set as rotary wall condition. Rotational speed n of three-phase asynchronous induction motor can be represented as
n=
60[s min]f p (1 − s )
(2)
Where Ck and Gb represent the generation of turbulence kinetic energy due to the mean velocity and buoyancy, respectively, YM denotes the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, C1ε, C2ε, C3ε, σκ and σε are default values from ANSYS, Sκ and Sε are user-defined source terms. 3. Experimental verification In order to verify accuracy of this model, temperatures at certain positions inside a rotary compressor under different conditions are measured. The measured rotary compressor is shown in Fig. 4. During measurement, the measured compressor using R32 as refrigerant is placed in the Calorie Tester. The test conditions and main parameters of the measured compressor are shown in Tables 2 and 3. Of the several conditions in Table 3, the high efficiency condition denotes the condition was designed to match the air conditioner with COP of 3.4–3.6. Since the size of rotary compressor used in room air conditioner is small and compact, the number of drilling holes on the
where f is frequency of A.C, p is pole pairs, s represents slip ratio. The temperature of the shell is higher than that of surrounding air, so the heat is transferred from shell to surrounding air. In the case of natural convection, radiation heat transfer has the same order of magnitude as convection heat transfer, so radiation heat transfer cannot be ignored. This boundary condition can be presented in the form
−λ
∂T ∂n
wall
= h ( Tw − Tf ) + εσ ( Tw4 − T∞4 )
(3)
Where λ is the thermal conductivity of shell, n is exterior normal direction of the boundary, h is the convective heat transfer coefficient, Tw is temperature of compressor shell, Tf is the temperature of the surrounding air, ε is emissivity of solid surface, σ is Stefan– Boltzmann constant, and T∞ is the temperature of surrounding wall. 2.4. Fundamental equations The heat conduction inside the solid in a three-dimensional Cartesian coordinate system should obey the following law:
∂ ∂x
⎛ ∂T ⎞ ∂ ⎜⎝ λ x ⎟+ ∂x ⎠ ∂y
∂T ⎛ ∂T ⎞ ∂ ⎛ ∂T ⎞ ⎜⎝ λ y ∂y ⎟⎠ + ∂z ⎜⎝ λz ∂z ⎟⎠ + q = ρc ∂t
(4)
Where T is temperature, λx, λy, λz is thermal conductivity of solid in x, y, z direction, respectively, t denotes time, q is the heat generation rate per unit volume. Due to the steady operation state of the compressor, the right hand item should be zero. During the actual operation of compressor, flow of refrigerant in the cavity of compressor is turbulent flow, so the standard k-ε turbulent model provided by ANSYS Fluent is adopted to model the flow of refrigerant. The turbulence kinetic energy k, and its rate of dissipation ε, can be calculated by the following transport equations:
∂ ∂ ⎡⎛ μt ⎞ ∂κ ⎤ ∂ + Gκ + G b − ρε − Y M + S κ (ρκ ) + (ρκui ) = ⎜ μ + ⎟⎠ ∂x i ∂x j ⎢⎣⎝ σ κ ∂x j ⎥⎦ ∂t ∂ ∂ ⎡⎛ μt ⎞ ∂ε ⎤ ∂ (ρε ) + (ρεui ) = ⎜ μ + ⎟⎠ ∂x i ∂x j ⎢⎣⎝ σ ε ∂x j ⎥⎦ ∂t ε ε2 + C 1ε (Gκ + C 3εG b ) − C 2ε ρ + S ε κ κ
(5)
(6)
Fig. 4. View of the measured rotary compressor. (a) Iron core. (b) Sectional view of the compressor.
Table 2 Main parameters of the experimental rotary compressor. Parameter
Value
Rated frequency (Hz) Input power (W) Refrigerating capacity (W) Height of compressor (mm) Inner radius of shell (mm) Outer radius of shell (mm) Height of stator winding (mm) Outer radius of stator winding (mm) Height of stator iron core (mm)
50 1173 + /−3‰ 3415.6 268.0 110.4 117.4 150.0 96.0 90.0
Note: The refrigerating capacity and input power of compressor are measured in ARI condition [12].
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Table 3 Test conditions. Item
High eff.
High eff. I
High eff. II
ARI
ASHRAE/T1 [13]
Condensing Temp. (°C) Evaporating Temp. (°C) Suction Temp. (°C) Subcooling Temp. (°C) Ambient Temp. (°C)
46.0 10.0 18.0 41.0 35.0
46.0 10.0 24.0 41.0 35.0
46.0 10.0 30.0 41.0 35.0
54.4 7.2 18.3 46.1 35.0
54.4 7.2 35 46.1 35.0
Note: The measurement accuracy is +/−0.3 °C.
Table 4 The measuring-point items. Serial number
Item
1 2 3 4 5 6
Gas above the stator winding Stator winding Tooth of the upper stator iron Middle of the upper stator iron Tooth of the lower stator iron Gas under the stator winding
shell is limited. Furthermore, it is not realistic to install too many measurement components inside the shell. Drilling a hole with large radius on the shell may damage the strength of compressor shell, so a corresponding nut is welded on the outer face of shell; the sheathed thermocouple is then screwed onto the nut. In order to measure the temperature of refrigerant gas above the stator winding, a sheathed thermocouple with thread (ST, STTT-T-ATB2C15D1M5F2L1PIT5W0S0) was used. A similar procedure is done to measure the temperature of refrigerant gas under the stator winding. Stator winding is the place where it generates the most heat, so a T-type thermocouple is inserted in the stator winding to measure the temperature of stator winding. Furthermore, to measure the temperature of iron core, two T-type thermocouples are inserted in the tooth and middle of silicon steel plate in the upper portion of iron core, as is shown in Fig. 5a, while a T-type thermocouple is inserted in the middle of silicon steel plate in the lower portion of iron core. The measurement accuracies of sheathed thermocouple (STTT-T), T-type thermocouple are +/−0.5 °C and +/−0.3 °C, respectively. The measuring-point items are listed in Table 4. After thermocouples are installed in measuring position, these thermocouple wires should be taken out of the compressor through copper pipes welded on the shell, and the copper pipes should be sealed by using glue. During the experiment, the measured compressor is in the condition of natural convection. The temperatures of measured points are obtained after the stable operation of the compressor. Fig. 6 shows the comparisons between simulated results and experimental data under different conditions. It can be seen that the simulated results fits the experimental data well. Furthermore, the temperature of stator winding is higher than that of other places, especially the temperature of stator winding under the ASHRAE/T1 condition, which is 8.3 °C higher than the maximum acceptable temperature of the most widely used B-class motor insulation class, so some methods to decrease the compressor discharge temperature must be adopted, such as wet suction by adjusting the expansion valve, and the usage of lubricant oils specifically designed for R32 [14]. 4. Results and discussion Fig. 7 illustrates simulated results of temperature distribution of stator winding, stator core, refrigerant gas under ARI condition.
Iron core
Sectional view of the compressor Fig. 5. Arrangement of measuring points. (a) High eff. condition. (b) High eff. I condition. (c) High eff. II condition. (d) ARI condition. (e) ASHRAE/T1 condition
Figs. 8–10 show simulated results of temperature distribution of motor-refrigerant in y = 0 mm, A-A, z = 100.0 mm plane, respectively. During the stable operation of compressor, the rotor with the upper and lower balancer rotates at rotational speed n. Affected by this, the refrigerant gas in upper and lower cavity of compressor rotates, and adequately mixes. Consequently, the temperature of refrigerant is relatively uniform. The highest temperature of outer face in motor-refrigerant model appears in the place contact with stator iron. This is understandable because the thermal conductivity of solid stator iron is higher than that of refrigerant gas. Hence, the heat transferred from stator iron to surroundings due to heat conduction is larger than the heat transferred from the refrigerant gas to surroundings due to heat convection.
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High eff. condition
High eff. I condition
ARI condition
High eff. II condition
ASHRAE/T1 condition Fig. 6. Comparisons between simulated results and experimental data under 5 conditions. Note: the bars indicate zero order (only measurement apparatus) uncertainty
Figs. 8 and 9 show simulated results of temperature distribution of motor-refrigerant in y = 0 mm, A-A plane, respectively. The total temperature variations of stator winding are small, less than 2.5 °C. Furthermore, in general, the temperature of stator winding in the same height is about 1.3 °C higher than that of stator iron. This is mainly because heat per unit volume in stator winding is higher than that in stator iron. Furthermore, the heat of stator winding cannot sufficiently transfer to stator iron due to the existence of the insulator, so it appears apparent temperature boundary, as shown in Fig. 10.
The temperature of stator iron decreases with the increase of the radius in a horizontal section, mainly because the outer edge of stator iron is in contact with the refrigerant gas and shell with colder temperature. Moreover, affected by cooling passage arrangement, the temperature of stator iron in the same horizontal plane is not identical. Due to the small clearance between stator and rotor, the refrigerant flow rate is low. Further, the refrigerant gas here absorbs the heat caused by the rotor aluminum losses, stator iron losses, and copper losses, hence, its temperature rise is relatively high, up to about 8 °C. While the flow rate of the refrigerant gas in stator cooling
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Fig. 7. Simulated results of temperature distribution of motor-refrigerant.
ducts is high, and the refrigerant gas at high temperature dissipates heat to surroundings air through shell, so its temperature rise is low. The two gas flows mix in the motor upper cavity, and then discharge from discharge pipe.
When the compressor reaches steady state, heat transfer among refrigerant, motor and ambient air reaches a balance and the temperature inside the rotary compressor won’t change. The heat balance for the model can be presented in the following form:
Fig. 8. Simulated results of temperature distribution in y = 0 mm plane.
Fig. 9. Simulated results of temperature distribution in A-A plane.
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temperature rises as suction temperature rises, and shell temperature and natural convective heat transfer coefficient will increase, accordingly, so natural convective heat transfer is bigger. The heat generated by motor under ASHRAE/T1 condition is closer to that under ARI condition, while the heat dissipation is bigger than the latter, so the heat absorbed by refrigerant is less. Influenced by the rotation of rotor, the refrigerant gas in the annular gap between rotor and stator flows upward spirally. Under ARI condition, the mass flow rate of refrigerant gas in the annular gap between rotor and stator is 0.00473 kg/s; on the other hand, the refrigerant gas in the cooling ducts between iron core and shell flows upward, the mass flow rate in the cooling ducts between iron core and shell is 0.00937 kg/s.
5. Conclusions
Fig. 10. Simulated results of temperature distribution in z = 100.0 mm plane.
Q loss = Q refri + Q amb
(7)
Q amb = Q conv + Q radi
(8)
where Qloss represents the heat generated by motor, Qrefri is the heat absorbed by refrigerant, Qamb is the heat transferred between the shell and ambient air, including and heat transfer by natural convection Qconv and heat transfer by radiation Qradi. Qrefri can be calculated by the following equation:
( Toutlet − Tinlet ) Q refri = mC
(9)
where m is the mass flow rate of refrigerant, C represents specific heat of refrigerant, and Tinlet, Toutlet is the temperature of inlet and outlet of refrigerant, respectively. Qrefri, Qconv and Qradi under 5 conditions are shown in Fig. 11. The ratio of radiation and natural convective heat transfer ranges between 1.1 and 1.4, so the radiation heat transfer cannot be neglected when calculating the heat dissipation of shell under natural-convection environment. Comparisons of the former 3 conditions and the latter two conditions show that the convection heat transfer increases with an increase in suction temperature. This is because the discharge
In this study, the temperature distribution for motor-refrigerant in R32 hermetic rotary compressor is obtained by using fluid– solid coupling method and computational fluid dynamics technology. Based on the simulated results presented in this paper, the following conclusions can be drawn: 1. The temperature of stator winding under the ASHRAE/T1 condition is 8.3 °C higher than the maximum acceptable temperature of the most widely used B-class motor insulation class, so some methods to decrease the compressor discharge temperature must be adopted. 2. The simulated results fit the experimental data well. Under these test conditions, the maximum error is less than 5 °C, and it appears in tooth of the lower stator iron under ARI condition. 3. The total temperature variations of stator winding within the scope of their height are small, in ARI condition, less than 2.5 °C. It indicates that a lumped method is accurate enough for the temperature of the stator winding. 4. It is remarkable that the temperature rise of the refrigerant passing though between stator and rotor is, with up to 8 °C, much higher than for refrigerant that passes through the cooling ducts (up to 1.7 °C). 5. The ratio of radiation and natural convective heat transfer ranges between 1.1 and 1.4 under 5 conditions, so the radiation heat transfer cannot be neglected when calculating the heat dissipation of shell under natural-convection environment.
Fig. 11. Comparison of heat transfer rate under 5 conditions.
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Nomenclature [2]
c f h I n p R s t T λ ε σ ρ μ m
specific heat capacity (J/ (kg•K)) AC frequency (Hz) convective heat transfer coefficient (W/ (m2•K)) phase current (A) rotational speed (r/min) pole pairs (-) phase resistance (Ω) slip ratio (-) time (s) temperature (°C) thermal conductivity (W/ (m2•K)) emissivity (-) Stefan–Boltzmann constant (W/ (m2•K4)) density (kg/m3) dynamic viscosity (Pa•s) mass flow rate (kg/s)
Subscripts w wall surface f fluid ∞ infinity x x axis y y axis z z axis References [1] S. Taira, A. Nakai, R. Yajima, 2010. Evaluation and Practical Use of Equipment Using New Refrigerant as an Urgent Countermeasure against Global Warming.In:
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