Thermal simulation analysis of outdoor cabinets in substations

2. Heat transfer mechanism

Generally, heat is transferred in three ways.

(1) Conduction. Conduction is the main method of heat transfer in solids. The amount of heat transferred is proportional to the thermal conductivity of the material, the temperature difference, and the area through which the heat is transferred, and inversely proportional to the length through which the heat is transferred.

(2) Convection. Convection is a method of heat transfer between a solid surface and a nearby fluid. The amount of heat transferred is proportional to the convection coefficient, surface area, and temperature difference between the surface and the fluid.

(3) Radiation. Thermal radiation is the process by which an object at a certain temperature emits heat in the form of electromagnetic waves. The magnitude of an object's thermal radiation is proportional to the surface area of ​​the object, the surface emissivity of the object, and the fourth power of the temperature.

The heat dissipation design of the enclosed outdoor cabinet follows the above principles, optimizes the cabinet structure, rationally arranges the internal components, and selects the appropriate heat dissipation method to achieve the purpose of heat dissipation of the internal components of the device.

3. Establishing a mathematical model

1. Governing equations

During the calculation process, the comprehensive effects of fluid and heat transfer must be considered, and the calculation basis is as follows.

In steady state: The continuity equation of the fluid is: where u, v, w are the velocity components in the x, y, and z directions.

The momentum equation is

Where p is the pressure and is the viscosity coefficient.

The energy equation is:

The simulation calculation is to solve three equations to obtain the u, v, w distribution of each point of the fluid as well as the P and T distribution when it is in a steady state.

2. Build a physical model

The size of the entire cabinet shell is 1000mm×600mm×550mm. There are 2 6U half-width chassis and 2 1U full-width chassis installed inside. 2 fans are used to force air cooling for the 2 half-width chassis. A fan is installed in the lower space inside the cabinet for forced convection of air. The chassis are arranged up and down. In order to make the simulation results more accurate, the internal component layout of the 2 half-width chassis is simulated and modeled according to the different heat dissipation modules on each printed circuit board. The printed circuit board is set as an insulating material. The heat of the components inside the chassis is mainly transferred to the air and the shell by convection and radiation, and the heat is then exchanged with the outside world through the outdoor cabinet shell. Considering that the fan will affect the internal fluid distribution, a guide cover is installed between the fan and the chassis during the structural design. The overall model of the system is shown in Figure 1.

4. Set up the simulation structure plan

Requirements that outdoor cabinets should meet and issues that need to be considered during thermal design.

(1) Equipment reliability requirements should be met.

(2) The thermal environment requirements for the expected operation of the equipment should be met.

(3) It should comply with relevant standards and specifications.

(4) A trade-off analysis should be conducted on cooling methods to ensure the lowest cost and highest reliability of the equipment over its life cycle.

(5) The maintainability of the equipment should be improved.

(6) The heat problems caused by solar radiation to the equipment should be considered.

(7) The deposition of various particles should be prevented to avoid increasing the thermal resistance of the equipment and reducing the cooling effect.

(8) Thermal transients caused by changes in working cycle, power, and thermal environment should be prevented as much as possible to minimize temperature fluctuations.

According to the requirements of thermal design and issues that need to be considered, after the outdoor environment, internal space and installation are determined, the only factor that can affect the heat dissipation of the entire system is the structural solution, that is, whether the outer shell is equipped with a sunshade, whether there is forced convection of internal and external gases, and whether there is a fan inside the enclosed space.

Based on the above, five options were determined for analysis.

(1) Single-layer structure, no ventilation, no fan, the chassis dissipates heat naturally.

(2) Single-layer structure, no ventilation, and the cooling fan is on the top of the chassis.

(3) Double-layer structure, no ventilation, and the cooling fan is on the top of the chassis.

(4) Single-layer structure, no ventilation, and the cooling fan is at the bottom of the chassis.

(5) Single-layer structure, with ventilation, and the cooling fan is at the bottom of the chassis.

5. Setting parameters

The environmental parameters for this analysis are shown in Table 1. The board distribution inside the 2U half-width device is shown in Figure 2, and the parameter settings are shown in Table 2.