Investigation On The Performance Of Steam Ejector In Thermal Desalination System On Ship

With rapid economic development,the shipping industry plays an important role in economic development.During navigation,ships need to consume a large amount of fresh water to meet the needs of personnel and equipment on board,especially for large ocean-going cruise ships.Carrying fresh water from land is unable to meet the needs of ships.As a result,most ships are equipped with desalination units.Thermal desalination is an effective method to produce fresh water from the waste heat of ships.In this paper,theoretical analysis and experimental investigation were carried out on the marine low-temperature thermal desalination system with steam ejector.The main research work includes:Firstly,a marine low-temperature thermal seawater desalination system was designed to use the waste heat of jacket cooling water and the mathematical model of the system was established based on the characteristics of waste heat from the jacket cooling water of marine diesel engine.According to the difference between seawater inlet temperature and evaporating temperature of evaporator,there are two operation modes of the system.The following conclusions were obtained by the numerical calculation.When the evaporating temperature and the seawater inlet temperature is unchanged,the water production for each unit of heat increases with the rise of the generating temperature.Under the same operating mode,the water production for each unit of heat increases with the rise of the seawater outlet temperature,when the generating temperature and the evaporating temperature remain unchanged.When the generating temperature is 70?,the evaporating temperature is 10?,and the seawater inlet temperature is 8?,the maximum of the water production for each unit of heat is 1.321×10-3 kg/kJ.Compared with traditional vacuum boiling water desalination system,the water production for each unit of heat was increased by 288.5%in this thesis.Secondly,the numerical simulation of the influence of the special-shaped nozzles steam ejector was carried out.According to the constant pressure mixing model of steam ejector,the steam ejector with special-shaped nozzles applied to high and low temperature heat source were designed.The special-shaped nozzles included three lobed nozzle,four lobed nozzle,five lobed nozzle,six lobed nozzle,round-to-square nozzle and rectangular nozzle.The influences of the special-shaped nozzles on the internal flow of the steam ejector under the conditions of high and low temperature heat source were studied.The following conclusions were obtained:With the number of lobes in the lobed nozzles from three to six,the entrained ratio and critical condensing pressure of the steam ejector gradually increased under the conditions of high temperature heat source.The length of primary shock wave of lobed nozzle became longer,and the vortex of the steam ejector mixing chamber weakened,and the flow state of the steam ejector with lobed gradually becomes circular,but the entrained ratio and critical condensing pressure is lower than that of the steam ejector with Laval nozzle.The state of the primary shock and the secondary shock of the outlet of the round-to-square nozzle and the rectangular nozzle the steam ejector working at high temperature heat resource are basically the same as that of the Laval nozzle.The flow field of steam ejectors with rectangular nozzle and the round-to-square nozzle gradually becomes circular as the primary flow enters the mixing chamber,the entrained ratio and the critical condensing pressure of three kind of steam ejectors are nearly similar to each other.When the steam ejector is working under low temperature heat resource,the length of primary shock wave at the outlet of round-to-square nozzle and rectangular nozzle become longer than that of Laval nozzle,and the entrained ratio and critical condensing pressure of steam ejector increase slightly.Finally,the experimental investigation of the steam ejectors with special-shaped nozzles were carried out.The influence of nozzle shape and working conditions on the performance of the steam ejector with special-shaped nozzles were investigated,when the generating temperature is 110??130?,the evaporating temperature is 5??20? and the generating temperature is 40??70?,the evaporating temperature is 15??25?.The following conclusions were obtained;The critical condensing pressure of the steam ejector with lobed nozzle is lower than that of the steam ejector with Laval nozzle and the COP of the steam ejector with six lobed nozzle is higher than that of the steam ejector with Laval nozzle under the high temperature heat resource.Moreover,the COP and the critical condensing pressure gradually increase with the increase of number of lobes.The COP of the steam ejector with rectangular nozzle is higher than that of steam ejector with Laval nozzle,but the critical condensing pressure of the steam ejector with rectangular nozzle is lower than that of steam ejector with Laval nozzle.The COP of the steam ejector with round-to-square nozzle is nearly as same as the the steam ejector with Laval nozzle,and the critical condensing pressure of the steam ejector with round-to-square nozzle is between that of the steam ejector with the Laval nozzle and the steam ejector with rectangular nozzle.