Experimental design and verification of heat transfer flow in a scaled cabin model under the influence of convective and radiative coupling

With the development of aircraft towards high altitude, high speed, and long endurance, the influence of aerodynamic and radiant heat on the flow and heat transfer in the cabin has increased. To explore the flow conditions in the cabin under the influence of ventilation, convection, and radiation coupling, a scaled model test was conducted for verification and simulation. According to the scaling principle, a small ventilation box with a scale of 1: 5 was designed to simulate the boundary conditions in grid computing methods by building ventilation, heating, and irradiation systems. The CFD results were compared with experimental results for analysis. The results show that the experimental results of the scaled model are comparable to the flow field of the prototype model, and the experimental measurement data can be used as a reference indicator for evaluating the temperature field inside the model cabin. The comparison between the simulation experiment and the simulation results verifies the applicability of numerical calculation methods to simulate the flow field in the passenger compartment and the rationality of the scaled test technical scheme.


Introduction
The high-altitude environment is characterized by low temperature, strong radiation, thin air, and rapid changes in wind speed.Therefore, to meet the maneuverability and lift requirements of aircraft, highspeed flight must be achieved.The aerodynamic heating caused by high-speed flight is transmitted to the crew cabin through the cabin wall structure, solar radiation enters the crew cabin through the cabin window glass, and the environmental control system adjusts the air parameters by supplying air to the cabin.Therefore, from the perspective of environmental control design, it is necessary to consider the influence of thermal load and ventilation measures on the temperature field and flow distribution in the crew cabin, thereby guiding the design of the environmental control system [1,2,3].
Scaling experiment is a commonly used method in engineering [4], and its basic principle is that the main data results measured by the model and prototype system in the experiment should be similar.Liu et al. conducted scaled model tests to predict the noise environment in the launch well below the missile and test the sound absorption effect of a certain material.Zhang et al. [5] built a small-scale experimental platform for underground ventilation to verify the rationality of the mathematical model for airflow heat transfer inside buildings under the effect of underground heat storage.Ren et al. [6] provided spatial

Introduction to prototype passenger compartment model
According to the thermal protection manual of the classic high-speed aircraft SR71, when it flies at a speed of Ma=3, the air-drag heating can cause the temperature of the aircraft's leading edge to reach 650°C and the temperature of the outer surface of the fuselage and wings can reach 287°C [7].According to the thermal control parameters of the XB-70 aircraft, in extreme heat environments, the surface temperature of the cabin can reach 350°C.After cooling by the cabin wall protective material, the inner wall temperature of the crew cabin can be reduced to 60 to 70°C [8].
Under the conditions of a total airflow rate of 300 kg/h, an inlet air temperature of 20°C, a wall temperature of 50°C, and a solar radiation of 800 / 2 , the temperature field distribution in the crew cabin is shown in Figure 1.The calculation results indicate that the temperature in the prototype passenger compartment is symmetrically distributed horizontally, with a trend of higher temperatures near the knees and decreasing temperatures from the chest and abdomen to the shoulders.

Similarity theory
A reduction test is often used as a verification method in engineering design.The basic principle is that the main test data obtained during the test should be similar between the model and the original system.As scaling models are often used in building ventilation research, calculation and analysis are conducted concerning the similarity criteria for airflow model tests in ventilated rooms.

Deducing and analysis of the number of flow similarity criteria
Dimensional analysis is often used to determine similarity.The Reynolds number of the flow in the prototype crew cabin can range from 6000-12000.Achieving the same Reynolds number requires a very high wind speed in the scaled box, which is difficult to achieve in experiments.According to selfsimilarity theory, the values given in different literature are slightly different, generally considered to be 10 3 ~10 4 , which suggests that the flow in the prototype crew cabin enters the self-similar region.Therefore, the model design does not need to achieve the same Reynolds number but only to ensure that the airflow in the scaled model also enters the self-similar region.When the inlet temperature is not equal to the temperature in the scaled model, the non-isothermal airflow through the ventilation opening will be affected by inertial force and effective gravity.The Archimedes number can be used to characterize the similarity of non-isothermal airflow [9].

Scale model design
According to the scale calculation, the size of the small experimental box is 30 × 24 × 28 cm 3 , and it contains a small humanoid heating element.A transparent piece with dimensions of 11. 5 × 11. 5 cm 2 is placed on top of the box to receive simulated solar radiation input.The front and back of the box are equipped with air intake and exhaust ports to simulate ventilation and heat exchange.The experimental schematic diagram is shown in the Figure 2: In the experiment, heating of the inner surface is achieved through a controllable temperature polyimide heating film.Different airflow rates are allocated through high-pressure air diversion and pressure regulation.Sunlight is simulated through a solar radiator.The overall setup is shown in Figure 3

The verification of the flow calculation inside the cavity
The internal flow of a cavity is typically validated using a three-dimensional ventilation cavity example.The model is shown in the figure.The height is H, the length L is 3H, and the width W is H. Isothermal air enters the ventilation cavity from the left-upper inlet and exits from the right-lower outlet.The inlet height is h=0.056H, the outlet height is t=0.16 H, and the Reynolds number based on the height H is 5000.The reference data is the jet experiment by Nielsen in 1978 [10].The computational mesh is a structured grid with encrypted wall surfaces.It employs a velocity entry boundary with a free-flow exit and has no sliding adiabatic boundaries on the top, bottom, left, and right walls.The model is shown in Figure 4.

Result analysis
During the experiment, the temperature of the head, abdomen, knees, and air outlet of the dummy were measured.The experimental results are recorded in Table 1.The variation of the temperature difference between the inlet and outlet with the change of inlet air flow at different irradiance is shown in the following Figure 6.When no solar radiation was applied, the temperature difference between the inlet and outlet was correlated with the total air intake, which was expected because, in the absence of solar radiation, the heat inside the box was constant and only depended on the box wall temperature and the heat generated by the humanoid.The amount of heat carried away by the air was constant.According to  = ̇  ∆, as ̇ increased, ∆ decreased.
Observing under different air intake conditions, the temperature of the glass's inner surface significantly rises with the increase of solar radiation.In contrast, the temperature of the head measurement point changes relatively little, and the temperature of the knee measurement point changes more significantly compared to the head.The knee surface receives more radiation than other humanoid parts, so the temperature rises significantly.The simulated longitudinal and lateral temperature distribution is shown below in Figure 7.The comparison of observed point temperatures is shown in Table 2.

Conclusion
The calculated temperature values are generally higher than the experimental values.Still, the temperature change trend presented by the calculated results is relatively consistent with the experimental results, with a deviation within 20%, which meets the requirements of engineering calculations.Because the bottom plate of the box was not heated in the experiment but was placed on the experimental table, the bottom was treated as adiabatic in the simulation process.Therefore, the heat inside the box in the simulation is not dissipated to the experimental table as in reality.It is reasonable that the calculated result is higher due to this process.In summary, the flow field in the scaled cabin box can be similar to that in the original cockpit, proving that the numerical calculation method is reasonable.

Figure 1 .
Figure 1.Temperature distribution of characteristic surfaces.

Figure 3 .
Figure 3. Physical diagram of experimental device.

Figure 7 .
Figure 7. Temperature distribution of the prototype cabin's characteristic surfaces.