introduction

Vehicle insulation performance is one of the important indicators for evaluating vehicle performance, and it is also one of the important basis for air-conditioning unit selection. In railway vehicles, the thermal insulation performance of the vehicle is represented by the comprehensive heat transfer coefficient K value, according to the “Test Method for Thermal Insulation Performance of Railway Passenger Cars” (TB/T1674-1993) or “Railway Vehicle Air Conditioning Part 2: Type Test” (GB/T33193 .2-2016), the K value is measured in the environmental laboratory using the thermal stability test method of heating inside the vehicle. Due to the existence of the heat capacity of the vehicle body, it often takes a lot of time to reach a stable state of thermal equilibrium between the interior of the vehicle and the external environment in actual tests. Therefore, in order to shorten the test time and improve the test efficiency, this paper studies the method of quickly testing the K value of the railway vehicle during the temperature rise stage, and uses this method to test the K value of a certain type of railway vehicle.

1 The principle of thermal stability test K value

The thermal stability method is used to test the comprehensive heat transfer coefficient of the vehicle, which requires the vehicle to be placed in a constant ambient temperature (such as a vehicle thermal laboratory), and an electric heater is used to heat the air inside the vehicle. After that, the K value of the vehicle is calculated by the formula (1):

In the formula: P is the heating power input into the vehicle under thermal stability state (w): F is the outer surface area or geometric mean area of the heat insulation wall of the vehicle body (m2): Tin is the average temperature inside the vehicle (°C): Tout is the average temperature outside the vehicle (°C).

Using the thermal stability method to test the K value of the railway vehicle, after starting the electric heater in the vehicle, the average temperature in the vehicle changes with time as shown in Figure 1.

Figure 1 The variation curve of the average temperature in the vehicle with time by the thermal stability method

It can be seen from Figure 1 that during the process of testing the K value of railway vehicles using the thermal stability method, the average temperature inside the vehicle first rises rapidly, then gradually stabilizes, and finally the temperature inside the vehicle and the environment outside the vehicle reach a stable state of thermal equilibrium. Among them, the determination of thermal stability has strict requirements. For example, the standard “Test Method for Thermal Insulation Performance of Railway Passenger Cars” (TB/T1674-1993) requires that the average temperature of the insulation wall of the vehicle be maintained at (22.5±0.5)°C, and the temperature difference between the average temperature inside the vehicle and the average temperature outside the vehicle be maintained at ( Under the premise of 25±1)K, the thermal stabilization time shall not be less than 8h, during which the fluctuation of the average temperature outside the vehicle and the temperature difference between the inside and outside of the vehicle shall not exceed ±0.5K, and the fluctuation shall not rise or fall monotonously. The power fluctuation shall not be greater than 3%, and the fluctuation shall not rise or fall monotonically. When using the thermal stability method to test the K value of the vehicle, due to the existence of the heat capacity of the vehicle body, it takes a lot of time to completely reach a stable state of thermal equilibrium between the interior of the vehicle and the external environment, and the entire test process generally takes 2 to 3 days.

2 Principle of heating method to test K value

Using the temperature rise method to test the K value of the vehicle, it is only necessary to record the temperature rise data of the average temperature inside the vehicle, and the K value of the vehicle can be calculated without waiting for the internal and external environments of the vehicle to reach a stable state of thermal balance.

When using the heating method to test the K value of the vehicle, the vehicle is also placed in a constant ambient temperature (such as a vehicle thermal laboratory), and an electric heater is used to heat the air inside the vehicle. Keep the heating power Qs input into the car and the average temperature 1out outside the car constant, then in the heating stage, only the average temperature 1in inside the car changes. Assuming that the instantaneous K value of the vehicle obtained by testing at this time is Ks, the formula for calculating the K value is as follows:

Middle: Qs is the constant heating power input into the car (w): Ks is the instantaneous K value of the vehicle in the heating stage [w/(m2 K)]: F is the outer surface area or geometric mean area of the heat insulating wall of the car body ( m2): 1in is the average temperature inside the car (°C): 1out is the average temperature outside the car (°C).

On the other hand, due to the existence of the heat capacity of the car body in the heating stage, part of the heat actually input into the car body is transmitted from the heat insulating wall of the car body, and the other part is absorbed by the car body, that is, the heating power Qs can be written as:

In the formula: Q0 is the heat transmitted by the heat insulation wall of the car body per unit time (J/s); Q1 is the stored heat absorbed by the car body per unit time (J/s).

Among them, Q0 is related to the real K value of the vehicle, which can be written as:

In the formula: K is the real K value of the vehicle [w/(m2·K)].

Q1 is related to the heat capacity of the car body, assuming that the average heat capacity of the vehicle is w, and the temperature change inside the car is d1 within dT, then Q1 can be written as:

Therefore, from formulas (2)~(5), we can get:

When the temperature inside the vehicle and the environment reach a steady state of thermal equilibrium, d1/dT=0, and Ks=K at this time. Record the heating curve of the average temperature inside the car, as shown in Figure 2. According to the formula (2), the instantaneous K value of the vehicle in the heating stage is calculated, and the curve of the instantaneous K value of the vehicle with time is drawn as shown in Figure 3.

It can be seen from Figure 2 and Figure 3 that with the increase of the average temperature inside the vehicle, the instantaneous K value of the vehicle becomes smaller, and gradually tends to the real K value of the vehicle. Take any two points P1 and P2 on the average temperature rise curve in the vehicle and the instantaneous K value curve of the vehicle, the corresponding average temperature in the vehicle is 11 and 12, the instantaneous K values of the vehicle are Ks1 and Ks2, and the corresponding recording times are T1 and T2. On the heating curve, draw a tangent line to two points, and the slope of the tangent line is the temperature change rate d1/dT at the two points.

Figure 2 Heating curve inside the car

Figure 3 Vehicle instantaneous K value change curve

According to formula (6), for points P1 and P2 respectively:

In formulas (7) and (8), there are only two unknown quantities, the real heat transfer coefficient K of the vehicle and the average heat capacity w of the vehicle. Therefore, the average heat capacity of the vehicle can be calculated as:

Substituting the average heat capacity w of the vehicle into formula (7) or (8), the real heat transfer coefficient K of the vehicle is obtained as:

3 Application of temperature rise method to test K value

Use the temperature rise method to test the K value of a certain type of railway vehicle. According to the requirements of the “Railway Vehicle Air Conditioning Part 2: Type Test” (GB/T33193.2-2016) standard, the vehicle is placed in the thermal laboratory, and the temperature measurement points inside and outside the vehicle are arranged. As well as the electric heater in the car, set and maintain the ambient temperature at about 5°C, the power of the electric heater is about 8800w, and record the temperature rise data in the car. Figure 4 is the heating curve of the average temperature inside the vehicle, and Figure 5 is the curve of the instantaneous K value of the vehicle during the heating phase.

Figure 4 The temperature rise curve of a certain type of railway vehicle

Figure 5 The instantaneous K value change curve of a certain type of railway vehicle

The temperature test data of the 9th hour and the 17th hour in the heating process were taken respectively, and the K value of the railway vehicle was calculated according to the principle of the K value of the above heating method to be 1.178w/(m2 K). The vehicle K value takes a total of 20 hours.

**Research and Application of Temperature Raising Method to Test K Value of Railway Vehicles**

According to the test requirements of the thermal stability method in GB/T33193, after waiting for the internal temperature of the vehicle and the ambient temperature to reach a stable state of thermal equilibrium, the K value of the railway vehicle was tested to be 1.191w/(m2 K), and the thermal stability method was used to test the K value of the railway vehicle. The value took a total of 53h. It can be seen that compared with the thermal stability method, the error of the vehicle K value obtained by the temperature rise method is only 1.1%, and the test time is less than half of the thermal stability method.

4 Epilogue

This paper analyzes the principle of testing the K value of rail vehicles by the temperature rising method, and uses the temperature rising method to test the K value of a certain type of railway vehicle. The results show that the temperature rise method has high accuracy in testing the K value of rail vehicles, and this test method only records the temperature rise process in the vehicle, and the K value of the vehicle can be obtained quickly without waiting for the temperature inside the vehicle and the environment to reach a stable state of thermal equilibrium.

Using the temperature rise method to test the K value of rail vehicles can shorten the test time and improve the test efficiency, and is suitable for some research projects that need to quickly test the K value of rail vehicles. In addition, in view of the phenomenon that the vehicle is prone to “false thermal stability” in the actual test, the temperature rise method can also be used to assist in judging the accuracy of the thermal stability test.