Heat Transfer Performance of Water-cooled Heat Sink for I&C System Controller Chip in Nuclear Power Plant

To improve the heat dissipation reliability of the nuclear power control system chip, the structure of its water-cooled heat sink is compared and analyzed. Three schemes of water-cooled heat sinks are proposed, and CFD analyzes the heat dissipation performance of the heat sink. The simulation data of the heat transfer coefficient, pressure loss, and heat sink comprehensive coefficient are compared. The results show that the average temperature of the bottom surface of the three heat sinks tends to be flat with the flow velocity increase, and the heat dissipation coefficient Nu is also the same. The S-type heat sink has a higher heat transfer coefficient and lower average bottom temperature. The pressure loss of the S-type heat sink is much higher than that of the other two schemes with the increased flow rate, and the system resistance is large. By calculating the comprehensive coefficient of the heat dissipation process of the three heat sinks and considering the heat dissipation capacity and other factors, it is considered that the circular fin heat sink can be the optimal solution.


INTRODUCTION
Nuclear power control system is very important to the safe operation of the whole nuclear power plant.The nuclear electrical instrument control system has many high heat consumption components.When the working junction temperature of components is increased by 10℃, its reliability will decrease by about 60% [1] .Data show that about 55% of the damage or failure of electronic components is caused by excessive temperature.Therefore, the heat dissipation design and research of chip units of nuclear power control systems are particularly important [2] .
The heat flux of electronic devices increases with the high frequency, high speed, density, and miniaturization of electronic devices.Air cooling technology combined with software simulation optimization analysis for high heat consumption electronic products has been widely applied in many companies and units at home and abroad [3][4][5] .Despite the low cost of air cooling [6] , the bottleneck of heat dissipation capacity is difficult to break through.The high heat transfer capacity of the water-cooling system makes water-cooling technology more and more applied.Therefore, three kinds of water-cooled radiator schemes are proposed in this paper, and CFD analyzes the heat dissipation performance of the radiator.The simulation data of heat transfer coefficient, pressure loss and comprehensive radiator coefficient are compared, and the evaluation conclusion is given.

Physical model Figures 1(A)~(C).
The cooling water is provided with pressure by the liquid pump.Then it reaches the inside of the liquid cooling radiator through the water inlet, flows through the fin, absorbs heat, and then flows out of the water outlet to take away heat.However, the circular fin column radiator is special, which uses the top jet into the radiator and then flows out of the side outlet.(1) Mass conservation equation (for incompressible fluids): Among them, ρ is density, t is time, and u is the velocity vector.
(2) Momentum equation: Among them, P is static pressure and f is mass force.
(3) Energy conservation equation: where p C is the specific heat capacity, T is the temperature, k is the heat transfer coefficient of the fluid, and t S is the internal heat source of the fluid, and the part of the fluid mechanical energy converted into heat energy due to viscosity.

Grid division
Due to the complex structure of the radiator, the main grid type is hexahedron.The number of grids generated by straight tooth radiators, S type radiators and round fin column radiators is 215 and 737, respectively.
Figure 2 Model mesh number

Boundary conditions and calculation methods
The outer wall is adiabatic, the fluid medium is pure water, and the inlet temperature is 20℃.
Compared with the five working conditions, the mainstream Reynolds coefficient is Re=10000, 20000, 30000, 40000, and 50000, respectively.The lower surface is the heat source, heat consumption Q =100 W, and flow field using a SIMPLE algorithm.

Parameter Definition
Reynolds number: where U is the fluid inlet mobility.D h is the inlet fluid diameter.
Average surface heat transfer coefficient: (2) where q is heating flux density; T w is the average temperature of the bottom surface; T f is the average fluid temperature.
where the P in is the average inlet pressure; P ou is the average outlet pressure [8].To quantify the comprehensive effect of heat transfer, the comprehensive coefficient of the heat exchanger is defined as follows: where Nu∞ and Δp∞ are Nu and pressure loss Δp, respectively, when Re =10000 of straight tooth radiator.

Influence of radiator fin style on bottom surface temperature
Figure 3 shows the bottom surface temperature cloud map of different radiators when Re = 50000.It can be seen from Figure 3 that the temperature distribution of the radiator bottom plate is quite different.Figure 3(A) shows the bottom surface temperature distribution of the straight-tooth radiator.The temperature is higher at the upper and lower edges of the flow passage.Figure 4 shows the curve of the average temperature of the bottom plate of the radiator changing with the change of Re.The average temperature of the bottom plate decreases gradually with the increase of Re, and the S-type radiator is the most obvious.However, with the increase of Re, the average temperature of the bottom plate becomes more and more gentle.The bottom plate temperature of the straight tooth radiator is significantly higher than that of the other two radiator forms, mainly due to the low fin utilization efficiency.Figure 5 shows the relationship between Nu and Re of different radiators.The heat dissipation capacity increases with the increase of Re, and the heat dissipation effect of the S-type radiator is the best, followed by the circular wing column type, and the worst is the straight tooth type.
Figure 4 The relationship between the average Figure 5 The relationship between Nu and Re

Influence of radiator fin style on fluid pressure loss
Figure 6 shows the relationship between the inlet and outlet pressure drop of different types of radiators and the number of Re.At the lower stage of Re, the resistance is close, while with the increase of fluid velocity, the straight tooth type and the round fin column type radiators show obvious advantages.Straight tooth type and round fin column type radiators always maintain low resistance.The resistance of the S type increases with the increase of Re.When Re = 50000, the maximum resistance of the S-type radiator is as high as 567.8 kPa, which is the most consistent with the estimate.

Analysis of comprehensive radiator coefficient
The comprehensive coefficient of the radiator represents a relative ratio between heat transfer and fluid resistance and can reflect the comprehensive effect of heat dissipation.The direct tooth radiator is the reference frame to compare the comprehensive relative coefficients.The results are shown in Figure 7.The comprehensive effect difference between the direct tooth radiator and the circular fin column radiator is small.In contrast, the S-shaped radiator significantly reduces the comprehensive effect of the radiator by more than 35%, among which the high resistance greatly affects the comprehensive coefficient.Considering the factors of temperature and efficiency, the circular wing column is considered the best scheme.

Figure 1
Figure 1 Three kinds of heat sink types and inlet-outlet styles2.2Governing equationThe model simulates fluid flow, and the medium is continuous, following the continuity, Naiver Stokes, and energy conservation equations.When dealing with turbulent heat transfer problems, corresponding additional equations are added to the three control equations for a solution, where the zero equation can usually achieve good results.The following are three sets of control equations and zero equations.(1)Mass conservation equation (for incompressible fluids):

Figure 3 (
B) shows the bottom surface temperature distribution of the S-type radiator.The low outlet temperature and high outlet temperature of the flow channel inlet are caused by the heating of the fluid after passing through the radiator.Figure 3(C) shows the temperature distribution on the bottom surface of the circular wing column.An obvious high temperature area at the diagonal corner of the water outlet is caused by insufficient heat exchange due to weak convection.(A) Straight Tooth Type (B) S Type (C) Circular Fin Colum Figure 3 Temperature field on the heat sink bottom

Figure 6
Figure 6 The relationship between Re and the pressure drop

Figure 7
Figure 7 The relationship between comprehensive coefficient and Re