ORC Direct Contact Heat Transfer Performance Of The Steam Generator And Its Optimization

Utilization of low temperature waste heat in industrial production process can not only solve the energy problem but also reduce the environmental pollution. Recently, most researchs focus to the conversion the low-temperature waste heat, low focus ratio solar energy or biomass combustion heat into electric energy via Organic Rankine Cycle (ORC). One of the key equipment of ORC is the traditional heat exchanger, which has the disadvantages of high heat transfer temperature difference, low heat transfer ability, large flow resistance, huge volume and high cost. Direct-contact heat transfer involves the exchange of heat between two immiscible fluids by bringing them into contact at different temperatures. It has many advantages over traditional heat exchange methods because of a simpler design, lower temperature driving forces, and higher heat transfer rate. When the high boiling point heat transfer oil is used as the heat transfer fluid and low boiling point refrigerant is used as cyclic working medium in ORC, the similar condiation as direct contact heat exchanger with continous phase fluid and dispersed phase fluid can be obtained. Therefore, it is expected to achieve better performance when a direct contact heat exchanger equipped in ORC system.In this thesis, the theory of single bubble and bubble swarm generatation and heat transfer has been studied. Based on digital image processing techniques and computational homology, the bubble swarm patterns was reseached to quantify the synergy of bubble swarm patterns and heat transfer performance. Simulation and optimization model of ORC direct contact evaporator was built, which would be significant in the application of direct contact heat exchanger equipments for ORC equipments with the recovery of low-temperature waste heat.(1) A novel physical geometry model of single bubble growth rate and heat transfer was proposed. The model was based on the energy equation included both convection and conduction heat transfer from the continuous liquid to the bubble with the geometry of bubble in the actual heat transfer process. The bubble radius ratio and heat transfer coefficient Nu had been found. And base on the drift flux model, the volumetric heat transfer coefficient of bubble swarm was built. This model was divided into a preagglomerative and a postagglomerative stage on the basis of an assumed maximum value for the dispersed phase volume fraction.(2) Experimental platform for testing the ORC direct contact evaporator was designed and established. A new environmental-friendly refrigerant of R245fa and THERMINOL(?)66heat transfer oil were used as dispersed phase and continuous phase in the heat transfer test by the ORC direct contact evaporator, respectively. Based on this platform, prediction precisions of the volumetric heat transfer coefficient of bubble swarm were checked. The results revealed that there was a favorable precision in the calculation with the average error of16%, which could meet the requirement of precision in engineering calculation.(3) Simulation model of ORC direct contact evaporator was built based on the synthesis of thermodynamics, heat transfer theory, hydrodynamics and exergy analysis. The thermal and structural parameters to affect the ORC direct contact heat exchanger performances were determined. This model was used to simulate the performance of ORC direct contact evaporator. The volumetric heat transfer coefficient, refrigerant steam generation, effectiveness and exergy efficiencies affcted of the independent variables on initial heat transfer temperature difference, refrigerant and heat transfer oil flow rate. The results showed that there existed a complicated nonlinear relationship between these independent variables and system performances. Thus, it is also necessary to optimize the system’s multiple variables simultaneously.(4) This paper proposed a novel method to quantify the synergy of bubble swarm patterns and heat transfer performance in a ORC direct contact evaporator by using computational homology. These patterns of bubble searm were treated in the Matlab software environment including the original image, top-hat transform, binarization, and open operation. Betti numbers were used to estimate the number of bubbles aggregating in flow patterns and to obtain the pseudo-homogeneous time. A simple linear model of a bubble swarm and the heat transfer performance of a ORC direct contact evaporator was constructed on the basis of experimental analysis, in which a new index (βt) was defined by the Betti number average as well as the pseudo-homogeneous time. A good fitting curve between βt and the volumetric heat transfer coefficient average was obtained with a correlation coefficient of0.95. (5) Optimization design method of ORC direct contact evaporator was established via the application of mathematical programming theory and genetic algorithm (GA). The indexes volumetric heat transfer coefficient of ORC direct contact evaporator was used to evaluate the system performance. The performances of ORC direct contact evaporator was effectively improved by means of the proposed optimization design method. The performances of ORC direct contact evaporator was raised up compared with the original design after optimized by this method. Used synergy of bubble swarm patterns and heat transfer performance as the constraint, the number of iterations decreased to55%and the result increased to0.3%comparing with the original optimization model.