Research On The Formation Mechanism And Influence Of Near-wall Flow And Heat Transfer Of Radiator-window

With the rapid development of social economy,fossil energy exhaustion and environmental deterioration have surged year by year.China is faced with severe challenges of resource conservation and green energy consumption.China’s central heating area is large,and energy consumption is huge.Further reduction of heating energy consumption has become a top priority.Distribution of the heat released by terminal heating units plays a significance role in indoor thermal environment and heating energy consumption.As the heat source in room,the heat released by radiator is mainly transmitted in the form of convective heat transfer,and the airflow field near radiator reflects the heat transfer states to a certain extent.Additionally,due to the fact that radiators are usually arranged under windows for natural ventilation and lighting,and near-wall airflow of radiator-window is complicated under the synergistic effects of radiators and windows.On the whole,few researches have revealed the effects of near-wall airflow of radiator-window on indoor thermal environment.This study takes the common civil heating room as the research object,and discusses the indoor radiator-window near-wall airflow flow and heat transfer characteristics and the influence on the indoor thermal environment construction through the methods of testing,experiment and simulation.Firstly,a typical radiator heating room is selected as the research target,and its indoor thermal environment is measured and analyzed.Indoor heat transfer model is established and indoor heat transfer between various parts is calculated.Through calculation,it is found that the heat transfer of window and exterior wall play a decisive role in the distribution of heat.Since the convective heat transfer is dominant for the inner surface of window,the convective heat transfer should be lowered to make the heat released by radiator more applied to heating people’s activity area and improve energy utilization efficiency.Secondly,the feasibility of thermal image velocimetry on measuring the near-wall airflow field over indoor vertical heating unit is analyzed through model experiments.According to actual arrangement of the radiator,an electronic heating film is attached to the lower part of one side wall of the model cavity to simulate the radiator.The near-wall airflow field above the electronic heating film is measured,and the measurement results of thermal image velocimetry and particle image velocimetry are compared.The experimental results show that thermal image velocimetry is reliable in measuring near-wall airflow field over indoor vertical heating unit,and the measurement results can reflect the main flow characteristics of air motion.What’s more,heat storage leads to lag between surface temperature fluctuations and near-wall turbulent motions.Although the measured time-averaged velocity is close to the actual value,the instantaneous velocity tends to be distributed in low velocity region due to the measured velocity depends greatly on surface temperature fluctuations.Based on spectral analysis of near-wall temperature,it is found that the velocity measured by thermal image velocimetry reflects the flow characteristics near boundary layer.Thirdly,through the experiments performed in artificial environment climate chamber.In a scenario where the radiator and the window are approximately on the same plane,the formation mechanism of near-wall airflow of radiator-window and its effect on indoor thermal environment are deeply analyzed.The results show that the heat plume generated by radiator and the downdraft near window form a mixed impact and jointly determine the near-wall airflow of radiator-window.When outdoor temperature is certain and the surface temperature of radiator exceeds a certain value,radiator plays a leading role in the distribution of airflow field.The air flow near the exterior wall between window and radiator tends to be stationary.Hot air is transported to upper area,and the vertical velocity distribution is uneven.When the surface temperature of radiator is constant and outdoor temperature decreases to a certain value,window plays a leading role in the distribution of airflow field,and the distribution of airflow field is more uniform.In addition,the convective heat transfer in the inner surface of window is quantitatively described based on the temperature difference between indoor and outdoor air and the maximum velocity near the window.Finally,based on the characteristics of indoor thermal environment and characteristics of near-wall flow and heat transfer of radiator-window,the upper temperature of radiator is introduced in this paper.When the temperature of lower part of window is close to that of the exterior wall between window and radiator,the average surface temperature of radiator is defined as upper limit temperature of radiator.When radiator heats the room at the upper limit temperature,air velocity near the exterior wall between radiator and window tends to zero,lots of heat is lost from window through convection heat transfer,and energy utilization rate is low.Based on this point,methods for predicting upper limit temperature of radiator based on the flow law and momentum balance are established.Response surface analysis method is used to explore the effects of outdoor temperature,average surface temperature of radiator,window heat transfer coefficient and exterior wall heat transfer coefficient on temperature difference between indoor air and window and operating temperature.Taking thermal comfort and energy utilization as dual objectives,the optimal average surface temperature range of radiator at different outdoor temperatures is explored based on the established function relation.This study analyzes the characteristics and formation mechanism of near-wall air flow and heat transfer of radiator-window and its effect on indoor thermal environment through a combination of field measurement,experimental study and numerical simulation.As a basic research,this study will help deepen the understanding of the influence of near-wall air flow and heat transfer of radiator-window on indoor thermal environment,and provide theoretical support for the creation of a more comfortable and energy-saving indoor thermal environment for heating.