Numerical Study on the Performance Characteristics of a Rotary Regenerator according to Design Parameters

Transcription

1 Nuerical Study on the Perforance Characteristics of a Rotary Regenerator according to Design Paraeters HYUN JOON CHUNG 1, DOWON CHA 1, HOON KANG 2, YONGCHAN KIM 2 1 School of Mechanical Engineering Korea University Graduate School Ana-Dong Seongbuk-Gu, Seoul KOREA 2 School of Mechanical Engineering Korea University Ana-Dong Seongbuk-Gu, Seoul KOREA haiba@korea.ac.kr, cdw0716@nate.co, kh-lab@hanail.net, yongcki@korea.ac.kr Abstract: - A desiccant air conditioning syste can control both teperature and huidity. A rotary regenerator adopted in the desiccant air conditioning syste perfors pre-cooling of the supply air into an evaporative cooler and pre-heating of the regeneration air into a desiccant rotor. The disk of the rotary regenerator is usually ade of etallic aterial such as aluiniu. Plastic aterial such polyethylene theraplate is also used to decrease weight because of its higher heat capacity and lower density than those of aluiniu. However, its adsorption characteristic can cause additional water vapor transfer. The water vapor transfer causes degradation of the perforance of the desiccant air conditioning syste because it increases the huidity of supply air. In this study, a nuerical odel considering heat and water vapor transfer by leakage and adsorption is developed and the perforance characteristics of the rotary regenerator using plastic aterial are investigated using the odel. The increase in the length of the rotary regenerator showed ore effect on the increase of both the heat and water vapor effectiveness than the increase in the diaeter. Key-Words: - Rotary regenerator, Heat transfer effectiveness,, Pressure leakage, Carryover leakage, Adsorption 1 Introduction A desiccant air conditioning syste integrates a desiccant wheel and evaporative cooler together, so it can control both teperature and huidity. A rotary regenerator, adopted in the desiccant air conditioning syste, perfors pre-cooling of the hot and dry air strea to the evaporative cooler and pre-heating of the cold and huid air strea to the desiccant rotor. The rotary regenerator consists of a rotating disk and casing. The disk is divided into hot and cold regions by the casing. As the disk rotates, heat energy is transferred to disk in the hot region and released to the air in the cold region. Water vapor can be also transferred fro the cold and huid air to the hot and dry air by leakage through the gap between the disk and the casing. Metallic aterial such as aluiniu (AL) has been ainly used to ake the disk, but plastic aterials such as polypropylene and polyethylene theraplate (PET) are getting attention to reduce weight and price because of its high heat capacity and low density. However, plastic aterials cause additional water vapor transfer because of its water adsorption characteristic. Because water vapor transfer leads to the increase in huidity of the air supplied to the conditioned space, the adsorption in the rotary regenerator using plastic aterial should be investigated for the design of the rotary regenerator. Kays and London [1] presented an epirical correlation to copute the effectiveness of a rotary regenerator. Wang et al. [2] nuerically investigated the effect of design paraeters on the effectiveness of a rotary regenerator using exergy analysis. Sarangi and Choowdhury [3] analyzed the entropy generation of a rotary regenerator. Shah and Skiepko [4] developed two separate odels for the heat and water vapor transfer by leakage and investigated the influence of leakage on theral perforance [4]. Chung et al. [5] investigated the effect of heat conduction resulting fro using plastic aterial. However, there is hardly any study on the effect of the water vapor transfer by adsorption of plastic aterial. In this study, a nuerical odel considering heat transfer and water ISBN:

2 vapor transfer by leakage and adsorption was developed. The effects of the design paraeters on the heat and water vapor transfer were investigated. 2 Modeling of the Rotary Regenerator Fig.1 shows the control volue of a rotary regenerator. The disk is divided into sall segents along the flow direction, and each segent is divided into sall segents again along the rotating direction. Under the assuption of adiabatic channels and unifor air flow distribution, all channels in a segent are considered to be identical. Thus, one channel with half thickness of atrix is chosen as a control volue. The energy and ass conservation equations are based on the odel developed by Jeong et al. [6], which are presented below. Mass conservations for the air strea and water vapor in the air strea are given as ( u ) ρa a Va + ja ( u W ) ρa a a Va + ja (1) (2) Energy conservation for the air strea is given as ( u h ) ρa a a Va + ( jh fg+ qs) A (3) Mass conservations for the atrix and water vapor in the atrix are given as Fig. 1 Scheatic of the control volue. Jeong et al. [6]. 2/3 h j Pr ua cp = ρ (7) j= Exp[ ln(re) a a K Sh d h ln(re) 0.04 ln(re) ] (8) ρ D = (9) The leakages are classified into pressure and carryover leakage and calculated using Equations (10) and (11), resepctivey, given by Skiepko [7]. ɺ = A L σ ρ N (10) co fr a ɺ = C A Y 2ρ p (11) p d g a ρ Vω ja ( X ) ρ Vω ja (4) (5) The friction factor is calculated using Equation (12) given by Kays and London [1]. f = Exp[ ln(re) ln(re) 0.04 ln(re) ] (12) Energy conservation for the atrix is given by ( h ) ρ Vω ( jh ads+ qs) A (6) The air-side heat transfer coefficient is calculated fro Equation (7). The Colburn j factor in Equation (7) is calculated using Equation (8) given by Kays and London [1]. The overall ass transfer coefficient is calculated using Equation (9) given by Fig. 2 shows the flow chart of the siulation progra. The teperature, huidity, and leakage flow rate of cold and huid air strea are assued. The governing equations are solved along the rotating and axial direction in turn. This procedure is iterated by changing the assued huidity until the assued and calculated huidity are balanced. This procedure is iterated again to balance the assued and calculated teperature by adjusting the assued teperature. After balancing the huidity and teperature, the leakage flow rate is calculated ISBN:

3 effectiveness, respectively. q ɺ h ɺ h q ɺ c T T u = = W v = W h,i h,i h,o h,o ( ) ax c,i p,c h,i c,i h,o c,i W W h,i h,i (13) (14) Fig. 2 Flow chart of the siulation progra. and copared to the assued one. If the balance is not satisfied, the assued leakage flow rate is adjusted and the procedure is iterated again. 3 Results and Discussion The geoetric paraeters and operating conditions are shown in Table 1. Equations (13) and (14) were used to calculate the heat and water vapor transfer Table 1. Geoetric paraeters and operating conditions of the rotary regenerator Diaeter Hot air inlet Cold air inlet Channel Teperature Huidity Teperature Huidity Shape Size o C 8.5 g/kg 35 o C 14.0 g/kg Triangle Perforance characteristics with rotating speed The perforance characteristics of the PET rotary regenerator were copared to those of the AL rotary regenerator. The diaeter and length of the rotary regenerator are 550 and 200, respectively. Fig. 2 shows the variations of heat and water vapor transfer effectiveness along with the rotating speed. The pressure leakage hardly varied and the carryover leakage increased alost linearly with the rotating speed. As a result, the water vapor transfer effectiveness of the AL rotary regenerator increased linearly with the rotating speed. However, the water vapor transfer by adsorption decreased with the increase in the rotating speed, so the increase rate of the water vapor transfer of the PET rotary regenerator gradually decreased. The heat transfer effectiveness of the PET rotary regenerator was lower than that of the AL rotary regenerator because voluetric heat capacity of PET, which was alost proportional to the heat transfer effectiveness, was 8.8% lower than that of the AL. Because of the additional heat transfer during the adsorption process, the heat transfer effectiveness of the PET Heat transfer effectiveness Rotor rotating speed (RPM) v PET AL Fig. 3 Variations of heat and water vapor transfer effectiveness with the rotor rotating speed. ISBN:

4 rotary regenerator was 6.7% lower than that of the AL rotary regenerator at the rotating speed of 9 RPM. The heat transfer effectiveness of the PET and AL rotary regenerators were and 0.922, respectively. 3.2 Perforance characteristics with design paraeters The perforance characteristics of the rotary regenerator with the variation of design paraeters were investigated. The volue of the rotary regenerator was varied fro 90% to 110% by varying the diaeter and length with the rotating speed fixed at 7 RPM. Fig. 4 shows the variations of water vapor transfer effectiveness by pressure leakage and adsorption, and Fig. 5 shows the heat and total water vapor transfer effectiveness. As the diaeter of the rotary regenerator increased, the velocity of the air flow decreased, resulting in v,p v,a Diaeter Volue deviation (%) Fig. 4 Variations of water vapor transfer effectiveness by leakage and adsorption. Heat transfer effectiveness v Diaeter Volue deviation (%) Fig. 5 Variations of heat and water vapor transfer effectiveness. the decrease in the friction factor. However, the gap between the disk and casing increased with the increase in the diaeter, so the water vapor transfer effectiveness by pressure leakage slightly increased. For the increased length, the pressure drop increased due to the increase in flow length, resulting in the increase in water vapor transfer effectiveness due to the pressure leakage. The decrease in velocity resulting fro the increase in the diaeter also caused the decrease in the heat and ass transfer coefficient, so the increasing rate of the heat and water vapor transfer by adsorption according to the increase in the diaeter was lower than that with the increase in length. When the volue of the rotary regenerator increased by 20%, the heat and water vapor transfer effectiveness increased by 5.0% and 11.0% with the increase in the diaeter and 11.8% and 13.8% with the increase in the length, respectively. 4 Conclusion A rotary regenerator odel taking into account the heat and water vapor transfer by leakage and adsorption was developed. The perforance characteristics of the PET rotary regenerator along with the rotor rotating speed were coparatively analyzed with those of the AL rotary regenerator. The heat transfer effectiveness of the PET rotary regenerator was 6.7% lower than that of the AL rotary regenerator even though the voluetric heat capacity was 8.8% lower. The perforance characteristics with the variation of design paraeters were also investigated. Changing the diaeter and length of the rotary regenerator influenced ore on the heat and water vapor transfer. Acknowledgeents This paper is supported by Korea Ministry of Environent as The Eco-Innovation Project and by the Huan Resources Developent progra(no ) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea governent Ministry of Knowledge Econoy. References: [1] W. M. Kays, A. L. London, Copact Heat Exchanger, McGraw Hill Co., [2] H. Y. Wang, L. L. Zhao, Q. T. Zhou, Z. G. Xu, H. T. Ki, Exergy Analysis on the Irrever- ISBN:

5 sibility of Rotary Air Preheater in Theral Power Plant, Energy, Vol.33, No.4, 2007, pp [3] S. Sarangi, K. Choowdhury, On the Generation of Entropy in a Counter-flow Heat Exchanger, Cryogenics, Vol.33, No.2, 1982, pp [4] R. K. Shah, T. Skiepko, Influence of Leakage Distribution on the Theral Perforance of a Rotary Regenerator, Applied Theral Engineering, Vol.19, No.7, 1999, pp [5] H. J. Chung, H. Jung, H. Kang, M. Kang, Y. Ki, Nuerical Study on the Effects of Transverse Wall Heat Conduction in a Rotary Regenerator, 22nd International Syposiu on Transport Phenoena, [6] J. Jeong, S. Yaajuchi, K. Saito, K. Kawai, Perforance Analysis of Four-partition Desiccant Wheel and Hybrid Dehuidification Air-conditioning Syste, International Journal of Refrigeration, Vol.33, No.3, 2010, pp [7] T. Skiepko, Indirect Estiation of Leakage Distribution in Stea Boiler Rotary Regenerators, Heat Transfer Engineering, Vol.18, No.1, 1997, pp ISBN: