| Efficient utilization of solar energy resources is an important way to achieve the "dual-carbon" goal.As one of the important equipment for solar thermal utilization,solar air heater is widely used in daily life.Jet impingement technology is applied to solar air heaters due to its strong heat transfer effect,that is,jet impingement solar air collector.However,since jet impingement solar air heater enhances heat transfer at the expense of pressure loss,it is necessary to balance the heat transfer efficiency of the collector with fan energy consumption.Therefore,studying the influence of local structure of the heater on the heat transfer and flow performance of jet impingement solar air heaters is of great significance for further improving the heat transfer efficiency of solar air collectors.The purpose of this study is to enhance heat transfer by reducing pressure drop or maintaining pressure drop while not decreasing the heat transfer efficiency of the heater.A new jet-impingement type solar air heater structure is designed for this purpose.The influence of jet direction,jet hole shape,absorber plate structure,and operating parameters on the heat transfer performance and collection efficiency were investigated.Two heater models,one with a rectangular protruding absorber plate structure and the other with a sine-like absorber plate,were studied.The specific design and experimentation work are as follows:(1)A solar air heater model with a downward jet impingement heating plate was established,and its flow and heat transfer characteristics were simulated and analyzed.The comprehensive heat transfer performance of this model was compared with a traditional solar air heater with upward jet impingement heating plate.The designed heater structure resulted in an average decrease of 2.4% in the average Nusselt number on the absorber plate’s surface,an average decrease of 9.05% in the friction coefficient and an improvement of 1.08% in the overall heat transfer coefficient.(2)The effects of nozzle shapes(circular,square,triangular,and rhombic)with the same area on heat transfer and flow in a jet impingement solar air heater were simulated and analyzed.The results showed that changing the geometric shape of the nozzle with the same area had little effect on the heat transfer performance results.Compared with square,triangular,and rhombic nozzles,the average improvement rate of comprehensive heat transfer performance obtained by air impinging on the absorber plate from a circular nozzle was3.33%,1.64%,and 1.72%,respectively.Taking into account the heat transfer performance and pump power consumption,the circular nozzle is more suitable for the jet impingement heater model.(3)The single-factor analysis method was used to study the effect of structural parameters on the performance of jet-impingement solar air heaters.The comprehensive heat transfer performance of the heater with protruding absorber plate was simulated and analyzed with respect to the relative height ratio of the protruding portion,the height-to-diameter ratio,the stream wise pitch ratio and the span wise pitch ratio.The comprehensive heat exchange performance of the heater with sinusoidal absorber plate was analyzed simultaneously in terms of the amplitude,period,flow stream wise pitch ratio and span wise pitch ratio.The results show that the optimal structural parameters for the heater with protruding absorber plate to achieve the best comprehensive heat transfer performance are the relative height ratio of 0.005,height-to-diameter ratio of 0.410,stream wise pitch ratio of 2.200 and span wise pitch ratio of 1.833.The optimal structural parameters for the collector with sinusoidal absorber plate to achieve the best comprehensive heat transfer performance are the amplitude of 1mm,period of 30,stream wise pitch ratio of 2.933 and span wise pitch ratio of 1.833.(4)Using response surface optimization,the geometric parameters of two types of absorber plate structures were deeply optimized to achieve the maximum comprehensive heat transfer performance as the objective function.The response surface optimization analysis was conducted to obtain the optimal combination parameters of the two heaters and the significant interaction effects of each parameter on the comprehensive heat transfer performance.The results showed that the heater with a protruding absorber plate structure achieved the maximum comprehensive heat transfer coefficient of 4.49 when the relative height of the protrusion was 0.025,the height-to-diameter ratio was 0.410,the stream wise pitch ratio was 2.933,and the span wise pitch ratio was 2.200.Compared with the optimal combination structure obtained by single-factor analysis,the optimized comprehensive heat transfer coefficient was increased by 2.98%.The heater with a sinusoidal absorber plate structure achieved the maximum comprehensive heat transfer performance of 5.96 when the corrugation amplitude was 1mm,the corrugation period was 60,the stream wise pitch ratio was 1.77,and the span wise pitch ratio was 2.19.Compared with the optimal combination structure obtained by single-factor analysis,the average comprehensive heat transfer coefficient was increased by 2.05% after optimization.(5)The heat transfer and flow characteristics of a collector model corresponding to the optimal combination of structural parameters were simulated and analyzed for two different absorber plate structures.The comprehensive heat transfer performance and collection efficiency of the two heater structures were compared at various Reynolds numbers.The results show that the sine-shaped absorber plate heater has better comprehensive heat transfer performance at low Reynolds numbers,while the heater with a raised absorber plate structure has better comprehensive heat transfer performance at high Reynolds numbers.In addition,compared to the collector with a sinusoidal absorber plate,the heater with a protruding absorber plate structure has a higher collection efficiency at the same Reynolds number.When the Reynolds number is 18,000,compared with the collection efficiency of the collector structure with a sine-shaped absorber plate,the collection efficiency of the heater structure with a protruding absorber plate is increased by 28%.(6)A model of a heater was established with the best comprehensive heat transfer performance for the sinusoidal absorber plate structure mentioned above,as well as a model of a heater with the highest collection efficiency for the protruding absorber plate structure.Firstly,the influence of Reynolds number on heat transfer characteristics,friction characteristics and comprehensive heat transfer performance was analyzed.Secondly,the performance of the heater in the same structure but with different jet directions was compared,and the comprehensive heat transfer performance of the two absorber plate structure heaters designed in this paper was compared with existing literature results.Finally,the influence of air inlet temperature on the comprehensive heat transfer performance of the collector was analyzed.The results show that compared with the heater structures of absorber plates with protrusions,multiple arc-shaped ribs or multiple V-shaped arranged protrusions,the average improvement rates of the comprehensive heat transfer coefficient of the jet-impingement solar air heater with sinusoidal absorber plate structure designed in this paper are 30.41%,20.84%and 32.67%,respectively.The comprehensive heat transfer performance decreases with the increase of Reynolds number,and increases with the increase of air inlet temperature.(7)A model of a heater with the highest collection efficiency for the absorber plate structure with protrusions mentioned above was established.Firstly,we analyzed the influence of Reynolds number on heat transfer characteristics,frictional characteristics and overall heat transfer performance.Secondly,we compared the performance of the heater in the same structure but with different jet directions.Finally,we analyzed the effect of Reynolds number and air inlet temperature on the collection efficiency of the convex-shaped absorber plate structure heater.The results show that the maximum collection efficiency of the heater with a convex-shaped absorber plate is 84.8%,and compared with non-jet impact flat plates,the designed structural collection efficiency in this paper was improved by an average of39.4%.The collection efficiency increases with the increase of Reynolds number,while it decreases with the increase of the air inlet temperature. |
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