Hava şartlandırma sistemlerinde kullanılan rotatif bir rejeneratörün modellenmesi ve simülasyonu

Abstract

Bu tezde, dönel tipteki bir rejeneratörün ( ısı ve nem değiştiricisi ) analitik olarak modellenmesi ve sayısal simülasyonu gerçekleştirilmiştir. Sistemden alınan bir kontrol hacmi için kütle ve enerji korunumu uygulamasıyla elde edilen diferansiyel denklemler, bilgisayar programıyla sayısal olarak çözülmek üzere sonlu farklar şekline getirilmiştir. Hesaplama belli bir zaman aralığı ile ilerletilerek, sistemdeki süreksiz ısı ve nem transferinin etkileri gözlenmiştir. Hesaplamalar sonucunda, belirlenmiş özelliklere sahip bir rejeneratördeki sıcaklık ve nem dağılımları elde edilecektir. Bu sonuçlar, kuşkusuz, sistem performansını yorumlama olanağı sağlamaktadır.

A heat and mass regenerator is a very special type of heat and mass exchangers. Heat regenerators were developed and used before heat and mass regenerators, so they formed the basis of the concept. A heat regenerator receives the heat from a hot flow, stores it temporarily and releases it to a cold flow. Similar working principle applies for mass regenerators in which mass ( i.e. humidity ) is transferred between two flows. Heat regenerators are used in various applications such as in heating plants, refrigeration systems, solar systems, especially in the recovery of waste thermal energy. There are two types of regenerators :A fixed bed regenerator consists of a storage bed through which hot and cold fluids travel alternatively in time. Firstly, heat and/or moisture is stored by the bed by one of the flows. After a certain time, the previous flow is shut off and the other flow is circulated in the bed, picking up heat and/or humidity. The other type of regenerators, which is the subject of this study, is the rotary regenerator. This device consists of a rotating cylindrical matrix that is separated into two sections along the axial direction of the cylinder. Hot fluid flows through one end while cold fluid flows through the other, forming a countercurrent. Parallel flow regenerators, being less efficient, are seldomly used. The assumed model for this study is of counterflow type. The matrix gains heat from the hot flow when rotating through the hot section, releasing heat to the cold flow when moving through the cold section. XII In air conditioning, rotary regenerators with special matrix materials are used for both heat and moisture recovery and for dehumidification of moist air. In the recovery case, the total heat regenerator has high matrix capacity for heat as well as moisture, while in the dehumidification case the regenerative dehumidifier has low matrix heat capacity and high matrix moisture capacity. The special matrix material for these applications mostly has hygroscopic or desiccant characteristics. Some examples of such materials are calcium chloride, lithium chloride, silica gel and activated alumina. When a solid material of this type is not available, it is possible to make use of aqueous solutions. For example corrugated asbestos paper impregnated with aqueous lithium chloride solution is used for regenerative dehumidifier and total heat regenerator matrices in U.S:A: The cost of generating thermal energy has continuously increased in the last decades. So the methods for recovery of waste energy have gained more importance. Rotary regenerator supplies cost savings by recovering heat and humidity when it is used in air conditioning systems. Regenerators have both advantages and disadvantages when compared to the direct-transfer type ( recuperators). The advantages are stated as follows : 1. A matrix type surface provides large transfer surface per unit volume, from 215 to 610 m2 per cubic meter. The most compact direct-transfer type heat and mass exchanger provides a ratio from 120 to 185 m2 per cubic meter. So, the regenerator is relatively effective for any given weight and surface limitations. 2. Fabrication of matrix type surfaces is not difficult whenever the material for the required application is available. 3. Because of the periodic flow reversals, the surface has the self- cleaning ability. In recuperative heat exchangers, the fluids flow in seperate channels and always in the same direction and same XIII deposits therefore accumulate and lead to decreased heat transfer and increased pressure drop. In regenerative exchangers the hot and cold fluids flow in opposite direction ( to obtain counterflow operation ) but alternately through the same channels. Therefore, any sooty deposits tend to be blown out by the succeeding cold fluid flow. 4. In the systems with humidification, the regenerator provides reduction of energy by recovering exhaust air moisture. When used for cooling, the regenerator reduces the cooling load which reduces the necessary size of cooling equipment ( compressor, cooling tower etc. ), cooling coils, pumps and piping. The major disadvantages of the rotary regenerators are the followings: 1. There is some mixing of the hot and cold fluids due to leakage and carry-over. If any purge sector is not used, there must be some mixing of the fluids because during the rotation of matrix, the trapped hot fluid in the channels is carried to the cold side and at the same time by the same mechanism, the cold fluid in the channels passes to the hot side. This type of leakage is called "carry-over leakage". This leakage decreases the effectiveness of the heat exchanger and also the toxic components in the hot fluid might pass to the supply air. ( If this contamination is undesirable, the carry-over of the exhaust gas can be eliminated by adding a purge section, where a small amount of clean air is blown through to the wheel and then exhausted to the outside ). 2. If the fluids are at the different pressures, as in the gas-turbine regenerators, the sealing problem becomes important. Due to sealing design limitations, the maximum pressure difference between hot and cold fluids should be less than four bars. However, for the regenerators which are used as air preheater in XIV boilers and furnaces, the sealing problem is not so important, because the pressure of the hot and cold fluids ( combustion air and flue gases ) are approximate equal to each other. 3. Restrictions in pressure drop in rotary regenerators necessitate a large flow area with the usual matrix surface. This may cause bulky ducting in the system. On the other hand, due to the high compactness, pressure losses are greater than those of the recuperators. The total heat regenerators with high heat and moisture efficiency have been found to give greater total heat recovery than sensible heat regenerators in hot humid and cold areas. In the formulation of this study, both heat and mass transfer will be considered, which is the case for a total heat regenerator. The objective of this thesis is to analyze the performance of a counterflow rotary regenerator. This analysis includes the simultaneous consideration of heat and mass transfer between both fluids and the rotating solid matrix. Transient temperature and humidity variations in the axial and circumferential directions of this system are considered in the governing differential equations. By including the circumferential variation terms, the effect of carry-over due to matrix rotation is also accounted for in the governing equations. Solutions to these governing equations by finite difference methods will yield both transient and steady state temperature and humidity profiles in the regenerator. These results are indications of system performance. In Chapter II of this study, the problem to be solved is further defined, including the necessary assumptions and modeling of the governing differential equations, derivations and bringing these equations to the final forms. It is described how the regenerator works and the equations of energy conservation for the flow and for the matrix will determine the XV temperature profiles in the device. On the other hand, the equation of mass conservation of water vapor will determine the concentration ( humidity ) profiles. Chapter III is a presentation of the development of finite difference equations and formation of the numerical solution system. In Chapter IV, a sample problem is constructed with proper assumptions, its solution with a computer program developed in FORTRAN 77 language and results are presented. Discussions, conclusions and suggestions for future study are given in the Conclusions Part. The results of computation have given temperature and humidity profiles in a regenerator of specified properties and they are presented in Appendix-A. The FORTRAN Program and its algorithm are given in Appendix-B and Appendix-C, respectively.