Experimental Study On Thermal Performance Of "flower" Pulsating Heat Pipe Solar Collector

Although flat-plate and vacuum tube solar collectors are widely used in the field of solar energy,they may also have such disadvantages as scale formation,tube blasting,and lower thermal efficiency.Since 2007,pulsating heat pipe(PHP)has been applied increasingly step by step in the field of solar collectors.Compared to the traditional solar collectors,PHP solar collector not only avoids their disadvantages,but also has the advantages of more simple structure,lower cost,considerable low-temperature resistance and rapid thermal response etc.Therefore,it is of great significance and practical value to research PHP solar collector.In allusion to the possible disadvantages of the traditional solar collectors,simultaneously in order to solve the problem that the condenser section is not of sunlight heat conversion function but occupies a part of ground area,In this thesis,a kind of flower-type PHP structure was proposed,and an experimental system of flower-type PHP solar collector was established.The theoretical analyses of heat transfer and heat loss of each part of the flower-type PHP solar collector were made,and heat collection performance of the experimental system of the flower-type PHP solar collector was tested,the effects of liquid filling ratio,cold water mass flow rate,the length of the condenser section and solar intensity on start-up and performance of the flower-type PHP were experimentally investigated.Moreover,the experimental method of PHP based on electromagnetic induction principle using magnetic liquid as a working fluid was discussed.The main research contents and achievements are as follows:1.In allusion to the problem that the condenser section is not of sunlight heat conversion function but occupies a part of ground area,a flower-type PHP structure was proposed.This kind of flower-type PHP has 16 U-turns,different from the traditional flat-plate type PHP.The evaporator section is evenly distributed on the disk radially,and a circular hole of 65 mm is opened in the middle of the circular heat absorber plate.The flower-type PHP is distributed circumferentially in the cold water pipe after bending at 90° as the condenser section.Flower-type PHP solar collector was based on the ordinary flat-plate solar collector structure with copper plate as a heat absorber,transparent glass plate as heat insulation cover,which made the flower-type PHP solar collector be of considerable bearing capacity and the reduction of the grand area,accorded with the current development trend of integration of solar energy and architecture.2.According to the requirements of the experimental research,the experimental system of the flower-type PHP solar collector was established.The whole experimental system was composed of the five parts such as flower-type copper PHP,cold water supply system,working fluid charging device,data collection system(including temperature,wind speed and radiation intensity),heat absorber plate,heat preservation and insulation.The heat absorber plate of the flower-type PHP solar collector converted partly solar energy into heat energy,whose part transformed into internal energy of cold water(useful energy),and the rest of the heat energy was transferred to the surrounding environment in the form of heat radiation,convection and conduction.The theoretical analyses of heat transfer and heat loss of each part of the flower-type PHP solar collector were carried out based on heat transfer theory.3.Heat collection performance of the experimental system of the flower-type PHP solar collector was tested,the effects of liquid filling rate,cold water mass flow rate,the length of the condenser section and solar intensity on start-up and performance of the flower-type PHP were experimentally investigated.The research results showed that:(1)The difference between the theoretical values and the experimental values was less than 8.0% based on the theoretical analyses of heat loss of the flower-type PHP solar collector,which could comparatively accurately reflect heat loss of each part of the flower-type PHP solar collector.(2)A series of experiments including performance test of the flower-type PHP solar collector etc were carried out outdoors.The experimental results validated that the flower-type PHP solar collector was of considerable start-up and operation performance.As the temperature of heat absorber plate was 50°C,and the radiation intensity was 500 W,the PHP started to operate.While it was a sunny weather,the accumulative total operation could be 7 hours,and the instantaneous thermal efficiency reached 50%.(3)The effects of different working fluids(i.e.water,ethanol and acetone)on the PHP were investigated.The research results showed that,in the same experimental conditions,heat transfer performance and wall temperature curves of the PHP varied greatly due to the difference in the thermal properties of the working fluid,in which the start-up and heat transfer performance of the PHP was the best while acetone used as a working fluid.Besides,it was found from the experimental research that the fluctuation extent of the wall temperature of the PHP was proportional to the specific heat capacity of the working fluid.4.For the problem that flower-type copper PHP cannot be visualized,an experimental method based on electromagnetic induction principle using magnetic liquid as a working fluid was presented.Considering the three conditions of the phase interface between liquid and vapor entering the coil,the theoretical model of the relationship between the flow velocity of the working fluid,the diameter of PHP and the contact angle(caused by the difference of wettability between the working fluid and the tube wall)and the induced electromotive force was established in the condition of entire entry coil of liquid column.In the precondition of not exceeding the critical pipe diameter of the PHP,the maximum induced electromotive force generated by the flower-type copper PHP device with the magnetic liquid could reach 13.191 m V theoretically.In addition,the calculation results showed that,the induced electromotive force could be effectively improved by increasing the velocity of the working fluid and the diameter of the PHP.In the same working conditions,the induced electromotive force was decreased with the increase of the contact angle between the liquid working fluid and the tube wall,but the influence of the contact angle on the induced electromotive force was quite limited.While the contact angle was increased from 40° to 70°,the induced electromotive force only was decreased by 5.37%.The research achievements of the flower-type PHP and its solar collector in this thesis are of certain guiding significance for the manufacture of flower-type PHP solar collector and ensuring its excellent thermal performance.Therefore,it will lay a scientific and technical foundation for the application of flower-type PHP in the solar collector.