Heat Exchange Performance Simulation And Improvement Of The Radiant-convective Air-conditioning Heat Exchanger With Parallel Pipes

The radiant-convective air-conditioning heat exchanger with parallel pipes (RCACHE) is one of the air conditioning terminal devices that provide heating and cooling for a building. The heat exchange between this terminal device and the indoor environment is mainly radiation and natural convection. The RCACH has a bright marketing future because the radiate heating method has the features of comfort and energy saving, and the natural convection way is safe, economic friendly and noiseless. Literature reviews show that the heat exchange performance of the RCACHE with a surface temperature of 30～45℃ has been studied by a series of experiences, and test results show that the heating capacity per square meter of RCACHE is 157.2～254.2W7m2, the heating capacity of the device is 533.2～862.1W, witch is not good enough compared with the traditional radiator. In this study, a simulating mode of the RCACHE has been built by CFD based on the theoretical analysis. The performance of the RCACHE under different working conditions has been simulated. And the performance improvement strategies were proposed and studied.The pipe surface of the RCACH are covered with longitudinal fins with height of 0.3mm, which belong to the category of micro-ribs. To simplify the calculation model, numerical simulations were adopted to study the strengthening effect of the micro-fin to the natural-convection heat transfer of a single pipe of the RCACHE, and the relations of the height of the fin and the heat transfer effect of natural-convection of RCACHE were analyzed. The heat transfer model of vertical cylinder with vertical fins has been built, and the corresponding performance of this model when the height of the vertical fin height ranges from 0.3-20mm was simulated. Simulation results show that the strengthening effect of the micro-fin to the natural-convection heat transfer of a single pipe of the RCACHE is weak. When the height of the vertical rib is 1.2mm, the average convection heat transfer coefficient (η) of the pipe compared to the light pipe only increased 1.8%; When the height increased to 5mm, the η of the ribbed pipe increased 16.1%. According to this feature,the pipes of the RCACH could be simplified to light pipe in the calculation model of the RCACH.Based on the above simulation results, a computational fluid dynamic(CFD) method is applied to simulate the RCACHE in the heating mode of a small room. In the simulation process the RNG k-ε turbulence model and DO radiation model were adapted to simulate the heat transfer between the RCACHE and room environment. And the heating capacity of RCACHE can be got from the simulation results, as well as the proportion of the radiant heat to the total transferred heat. To verify the feasibility of the above numerical model, a corresponding testing system of RCACHE has been set up, and experiments under the same conditions to the numerical model have been conducted. The heating capacity of RCACHE was calculated and analyzed based on the test results. The results of simulations and tests were compared, and comparison results show that they agreed with each other very well, the maximum error does not exceed 10%, which indicates that the numerical model is capable for predicting the performance of RCACHE.To increase the heating exchange capacity of RCACHE, two strategies, the increase of the number of heat exchange pipes to form a compact structure and the increase of the airflow disturbances at the equipment surface, have been suggested. According to the two strategies, two kinds of numerical models, the model with mode heat exchange tubes and the model with went pipes, have been built based on the previous numerical model of RCACHE. And the performance of the new built two kinds of numerical models was performed. From the simulations of the RCACHE with two rows of pipes, the total heating capacity increased 53.5% and the heating capacity per square meter increased 33.8% than the RCACHE with existing structure. In the simulation of the second kind of numerical models, the influences of the wind velocity and direction at the surface of the device on the strengthening effect of the heat transfer of RCACHE were got. The total heating capacity of the RCACHE with blowing air from one side increased from 2.2% to 53.9%, when the wind velocity at tuyere varied from 0.8m/s to 8m/s.On the premise of that the wind velocity satisfy the indoor thermal comfort request, the best wind velocity is recommend to 2m/s.