| Evaporative cooling technology is a kind of cooling mode using water to cool air,which has the outstanding characteristics of renewable and sustainable development.At present,the main research directions of evaporative cooling technology include improving the cooling efficiency of evaporative cooling system,reducing the supply air temperature and enhancing the sensible heat treatment capacity.However,there is a common weakness in different forms of evaporative cooling,that is,the air supply parameters and cooling capacity will change with the fluctuation of outdoor ambient air parameters.The evaporative cooling system assisted with mechanical refrigeration system can maximize both their advantages and make up for their disadvantages.It therefore presents a promising future of application.Firstly,in view of the shortcomings of the traditional evaporation cooling system,the temperature of the supply air treated by the dew point indirect evaporative cooling heat exchanger tends to the dew point temperature under ideal condition,and based on the comprehensive consideration of the cooling performance,equipment practicability,floor area and other practical factors of the heat exchanger,a new type of counter flow dew point indirect evaporative cooling heat exchanger is proposed.It has been studied theoretically and experimentally.Then,based on the above research,the study on the dew point indirect evaporative cooling and mechanical refrigeration composite air conditioning system is carried out from the aspects of experiment and numerical simulation.The main content is as follows:Firstly,by selecting the reference point of outdoor state reasonably,the models for the analysis of direct evaporative cooling and indirect evaporative cooling heat exchangers are proposed,and the thermal conditions of six typical cities are analyzed.Through analysis,it is found that the exergy efficiency ratio is the largest in Urumqi and Lanzhou in dry areas,and the smallest in Shanghai and Guangzhou in high humidity areas.The exergy efficiency ratio of dew point indirect evaporative cooling in dry area is 1.8 times that of ordinary indirect evaporative cooling and 1.2 times that of direct evaporative cooling.Compared with other evaporative cooling heat exchangers,dew point indirect evaporative cooling heat exchanger has the largest temperature drop and the highest exergy efficiency ratio,so it has better thermal performance.Secondly,the experimental systems of cross flow dew point indirect evaporative cooling heat exchanger and counter flow dew point indirect evaporative cooling heat exchanger are designed and established.According to the change of air wet bulb temperature in different regions of China,eight typical climatic conditions are simulated by air pretreatment device.The cooling performance of the two systems is experimentally investigated,and the influence of intake air flow and working-to-intake air ratio on the cooling performance are analyzed.The cooling performance under eight typical climatic conditions is also obtained.Under the test conditons,the dew point effectiveness of designed counter flow dew point indirect evaporative cooling heat exchanger is 0.64-0.77,and the cooling capacity is673-1390 W.Thirdly,based on the energy and mass conservation equations,a mathematical model describing the heat and mass transfer process of the counter flow dew point indirect evaporative cooling heat exchanger is established,and the reliability of the model is verified.According to the above mathematical model,the effects of intake air dry bulb temperature and relative humidity,primary air velocity,secondary air to primary air flow ratio,channel length and channel height on cooling capacity,exergy destruction,dew point efficiency and exergy efficiency ratio are calculated and analyzed respectively,and the appropriate ranges of parameters are obtained.The results of numerical simulation show that the larger cooling capacity and exergy efficiency ratio can be obtained when secondary air to primary air flow ratio is 0.3-0.4,the channel height should be set to 3-5 mm,and the channel length should be in the range 1.0-1.5 m.Fourthly,component models for mechanical refrigeration system simulation are established.In the process of model establishment,the general state equations are used to calculate the thermodynamic properties of refrigerants,which is suitable for many substances and simple in form;the compressor thermodynamic model is a lumped parameter steady state model;the capillary model is calculated by approximate integration technology,and the condenser and evaporator are calculated by one-dimensional inverse method.Mathematical model of flow distribution parameters.Combining the above mathematical models with the dew point indirect evaporative cooling heat exchanger model,a general simulation platform is established,and the influence of outdoor meteorological parameters on the cooling capacity and COP of the composite air conditioning system is analyzed.The simulation results are verified by building the experimental platform of the composite air conditioning system.Fifthly,in the guidance of the above theories and methods,the economy and energy saving of composite air conditioning system in Urumqi,Harbin,Beijing,Shanghai and Guangzhou during the cold season(June-September)are studied.Firstly,the cooling hours of Urumqi,Harbin,Beijing,Shanghai and Guangzhou are analyzed.The results show that the cooling hours from more to less are Guangzhou,Shanghai,Beijing,Urumqi and Harbin,and in the air conditioning season,the dew point indirect evaporative system can provide 33.9–100% of cooling load for the selected cities.Then,the initial investment cost and annual operation cost of dew point evaporative cooling system and mechanical refrigeration system are given,and the payback period of investment and the total cost savings during the service life of the composite system in each city are obtained. |
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