Modeling And Validation Of Dust Particle Deposition On Fin-and-Tube Heat Exchangers

Fin-and-tube heat exchangers are widely used in a variety of applications in the airconditioning,refrigeration and process industries.The air-side performance of fin-andtube heat exchangers adopted in room air-conditioners will decrease after long-term operation due to the coverage of particulate fouling.The accumulated particulate fouling reduces the air flow area and increases the air-side thermal resistance,causing a significant deterioration of the long-term performance of heat exchangers.The consumers not only care about whether the energy efficiency of a new air-conditioner statisfy the national standards,but also care about whether the energy efficiency of airconditioner will decrease seriously after long-term running.In order to quickly evaluate the long-term performance attenuation of heat exchanger and improve the long-term performance,it is important to set up an accelerated testing method,establish an evaluation standard and optimize the heat exchanger structure.However,there are some deficiencies on the long-term performance study of heat exchangers,including the acceleted particle deposition simulation method,the quantitative influence of deposited particles on heat exchanger performance,and the analysis method of the effect of deposited particles on heat exchanger performance.The purpose of the present study is to numerically and experimentally investigate the dry particle behaviors and wet particle behaviors on fin surface of heat exchanger,and to propose an analytical model of particle-deposited fin efficiency for predicting the heat transfer rate of fouled heat exchanger.1)A dry particle deposition model has been developed.The dry particle deposition process includes particle transportation,particle-fin impact and particle-fouling layer impact,and the dry particle deposition rate on fin surface can be predicted by the integrated numerical model which comprises the sub-models of these three processes.The sub-model of particle transportation can be developed by introducing particle motion equations on the particles,which can be calculated by integrating force balance on the particles.The sub-model of particle-wall impact can be developed by establishing the sticking criterions of incident particles and the transformation criterion of impact type.The sub-model of particle-fouling layer impact can be developed by establishing the sticking criterions of incident particles and the removal criterions of bed particles on the fouling layer.2)A wet particle deposition model has been developed.The wet particle deposition process includes the moist air dehumidification,the particle motion and the particle capture,and the wet particle deposition rate on fin surface can be predicted by the numerical model which comprises the sub-models of these three processes.The submodel of moist air dehumidification can be developed by deducing the mass transfer equations at the fin surface and the droplet surface,and can be used to predict the condensate droplet formation and growth.The sub-model of particle motion can be developed by introducing particle motions equations on the particles,which can be calculated by integrating force balance on the particles.The sub-model of particlecapturing efficiency can be developed by establishing the critical deposition rate in the stage of pure water adhesion,and establishing the sticking criterions of incident particles and the removal criterions of bed particles in the stage of particle-wet fouling layer impact.3)The dry/wet particle behaviors on fin surface of heat exchangers as well as the influence of deposited wet particles on the air-side heat transfer performance have been experimentally investigated,and the dry particle deposition model and wet particle deposition model have been validated.Two visualization experimental rigs have been established respectively for dry particle deposition and wet particle deposition.The images of dry/wet particle deposition process on the fins and tubes of test samples have been captured by the camera,and the effects of fin structure parameters and operation condition parameters on dry/wet particle deposition weight as well as the influence of depositied wet particle deposition on air-side heat transfer rate and pressure drop have been analyzed.The experimental results of dry/wet particle distributions as well as the particle deposition weithgt have been compared with the predicted results based on the dry/wet particle deposition model.The comparison results show that the simulation results of the predicted dry/wet particle distribution on wavy fin surface agree well with the experimental images,and the deviation between the simulation results of the predicted particle deposition weight and the experimental results are within ±20%.4)An analytical model for predicting the particle-deposited fin efficiency has been developed.The particle-deposited annular fin is taken as the basic fin type for the particle-deposited fin efficiency modelling,and the modelling approach is to simplify the fin as two two-dimensional coating layers on a one-dimensional clean fin firstly,and then to calculate the temperature distributions on both the clean fin and coating layers.The model of particle-deposited fin efficiency of annular fin can be extended to the calculation of rectangular fin with circular hole by the way of equivalent radius transformation.The present model is capable to precisely predict the heat transfer rate of fouled heat exchanger with multi-fins when the two input parameters of fouling layer thickness and convective heat transfer coefficient are given.However,when these two input parameters are unknown,they may be estimated by the method suggested in this paper,and the errors of the predicted heat transfer rates caused by the inaccurate estimation are acceptable in engineering applications.