Theoretical And Experimental Study On Flue Gas Flow And Heat Transfer In Rural Residential Heating System

As the main heating structure in northern China rural areas,heating flue has a history of more than one thousand years.Residents use the smoke produced by biomass combustion as heat transfer medium to flow in the heating flue to improve the indoor thermal environment.However,at present,th e construction and operation of rural heating flue system are mostly based on the experience of village craftsmen,which often lack systematic design and operation basis,and lack of guidance strategies to strengthen the flow and heat transfer process of rural heating flue.In this paper,the flow characteristics and heat transfer enhancement of heating flue are studied,which provides the basis for the actual construction design and efficient operation of rural flue.Based on the analysis of the flow characteristics in rural heating flue,this paper analyzes the flow characteristics and laws in three modes: downwind forward flow,upwind forward flow and upwind reverse flow.In downwind forward flow,both thermal pressure and wind pressure can promote the fl ow of flue gas,and the flow is more sensitive to the change of wind pressure;In the case of upwind and forward flow,the restraining effect of wind pressure on the flow should be considered to meet the corresponding constraint conditions;The smoke will flow into the room in the reverse direction when it flows against the wind,which should be avoided.Therefore,in the actual design and construction process of the heating flue,the outlet of the heating flue should be placed in the leeward of the roof to form a negative pressure with the surrounding environment at the outlet,so as to ensure that the flue gas flow is in the downwind positive flow;If the flow is upwind and forward flow,the critical height of avoiding reverse flow should be met in the des ign of heating flue to prevent the occurrence of upwind and reverse flow.The flow and heat transfer process of flue gas in vertical heating flue with different structures was studied experimentally.The results show that: with the increase of baffle number of heating flue,the heat transfer effect will gradually enhance.On the premise of ensuring the natural flow of flue gas,when the number of baffles increases from 2 to 8,the heat transfer effect is improved by23.48%.When the thickness of flue increases from 0.1m to 0.3m,the heat transfer efficiency of flue gas flow decreases by 35.64%.The experimental results provide technical parameters for the optimal design of heating flue structure.The flow and heat transfer process in the horizontal flue of a typical rural house in Wujia village of Yujia town was tested.The results show that the temperature of the flue gas in the heating flue varies greatly,and the average temperature difference between the inlet and outlet is 173.5℃.The maximum temperature of flue gas outlet is 106.6℃,and the minimum temperature is 77℃.The heat loss of flue gas is large,which has great energy saving potential.According to the experimental results,the changes of the density,specific heat,viscosity and thermal conductivity of biomass flue gas with temperature were considered.The convection radiation coupled heat transfer model of flue gas in heating flue and its solution method are established.Compared with the experimental results,the average error of wall heat flux is 4.73%,and the average error of wall temperature is 5.24%.The average error of wall heat flux and temperature of horizontal heating flue is 6.34% and 9.12%,which proves the correctness of the model.The convection and radiation heat transfer characteristics of flue gas flow in heating flue are simulated.For the vertical heating flue,the wall heat flux will decrease with the increase of the wall thickness.Therefore,on the premise of ensuring the wall strength,the wall thickness inside the room sho uld be reduced;Increasing the thickness of insulation layer by 20 mm will increase the wall heat flux by 8.5%.After that,the wall heat flux caused by increasing the thickness of insulation layer is basically unchanged.In practical application,combined with economic considerations,it can be considered that 20 mm is the best thickness of insulation layer.For the vertical heating flue with the height of 3m,when the height width ratio of the flue is between 10-20,the heat transfer effect is the best.For the horizontal heating flue,increasing the wall thickness will reduce the wall heat flow,and the heat transfer effect is better when the wall thickness is 0.1m.For 3m horizontal heating flue,when the length height ratio of the flue is between 25-50,the heat transfer effect is better.In order to further optimize the internal structure parameters of heating flue,based on the field synergy theory,a comprehensive evaluation factor was proposed to characterize the key structural indexes of the flue to e nhance the heat transfer effect.The influence of key structural parameters on heat transfer and flow resistance characteristics of rural heating flue was studied by numerical analysis method,and the structural parameters of rural heating flue were optimized.When the number of baffles per row is 9,6,5 and 4 respectively,and the ratio of baffle width to flue width is 0.075,0.075,0.15 and 0.075,the comprehensive performance of point structure,in-line structure,insert structure and turn back structure is the best,The results can be used to guide the internal structure design of heating flue.Based on the experimental results and simulation results,a new combined system of heating kang and flue wall is proposed,which expands the application mode of heating flue in rural houses.The heat transfer process of rural residential buildings is modeled and calculated,and the application effects of different flue heating systems,different walls and different regional conditions are studied.The results show that the average indoor temperature of the house is increased from 12.54℃ to 19.91℃ after using the combined system,which proves the good applicability of the combined system.The research work of this paper not only provides theoretical guidance and technical support for the design and application of heating flue in rural residential buildings,but also provides design basis for rural residential buildings with flow heat exchange heating by heating flue to achieve the goal of strengthening heat exchange.In addition,the proposed flue type composite wall can also enrich the application mode of rural residential heating flue,and provide new ideas and methods for energy-saving design of rural residential heating flue.