تجزیه و تحلیل تجربی و حساسیت گرمایش هوای دوار برای بازیابی حرارت دودکش گاز

Energy saving is one of the key issues, not only from the viewpoint of fuel consumption but also for the protection of global environment. A rotary regenerator (also called an air preheater or a heat wheel) is a sizeable porous disk, fabricated from some materials having a fairly high heat capacity, which rotates between two side-by-side ducts; one for the cold gas; the other for the hot stream. Its application is in a wide range of temperature waste heat recovery systems. In this work, a rotary regenerator is simulated by solving a developed mathematical model and optimized with the experimental design method. In this method, the effect of dimensionless parameters on the effectiveness of rotary heat exchangers was investigated. Numerical results were obtained by solving continuity, momentum and energy equations, and a two-step, predictor–corrector procedure is used. Experimental results are obtained by using a lab-scale rotary type regenerator and factorial design of experiments was performed for the analysis of the data. The simulation results have been compared with the experimental data and good agreement has been obtained.

Rotary regenerators are exclusively used in gas-to-gas heat transfer and mainly in waste heat recovery applications. They consist of a rotor usually made of corrugated materials which rotates at very low speeds with a constant fraction of the core facing partially for the hot and cold fluids. The heat transfer surface area or flow passages are generally made of metals with cellular structure. When the hot gas flows over the heat transfer surface (through flow passages), the thermal energy is stored in the matrix wall. During the cold gas flow through the same passages later, the matrix wall delivers the thermal energy to the cold fluid. Thus, heat is not transferred continuously through the wall as in a direct transfer type exchanger (recuperator), but is alternately stored and rejected by the matrix wall. Hausen [1], obtained a simple approximated solution for a regenerator heat transfer model in a special condition (C*=1 and (hA)*=1) using the finite difference technique. Creswik [2], extended this method for the unsteady state condition. Lambertson [3], presented a complete solution of the model for the hot and cold periods when C*=1. Kays and London [4], corrected his correlation for C*<1. Bahnke and Howard [5] developed an axial conduction model for the regenerators and solved the model using the finite difference method. Razelos [6], developed an approximated method of solution for C*<1. Djuric [7], considered thermal efficiency for the unsteady state condition. Leong and Toh [8], developed a software for the regenerator simulation using the ε_Ntu,0 method. Beck [9] extended Bahnke and Howard’s solution with matrix surface effects on the regenerator performance. Shah [10], investigated the effects of carry-over leakage on the regenerator performance and finally Buyukalaca and co-workers [11] correlated the effect of rotation on the regenerator performance. The mathematical model developed by Oregan [12], has been extended and solved in this work. The results of the model were used for the experimental analysis of the regenerator system. An explicit method of solution using two-step predictor–corrector procedure has been used to solve the transient heat transfer mathematical model.

In the present study, a rotary regenerator was simulated by solving a developed mathematical model and optimized with the experimental design method. In this way, the effect of dimensionless parameters on the effectiveness of rotary heat exchangers was investigated. There are only three main parameters affecting the regenerator efficiency. These are A=rotational speed, B=hot air velocity and C=cold air velocity. Statistical calculations were performed and ANOVA table for each of the parameters and their interactions were prepared and F values were compared. By comparing F values, it is cleared that B and C parameters and their interactions have significant effects on the efficiency of the regenerator. Finally the mathematical model is used to obtain the optimized parameters for the regenerator.