Pressure drop and heat transfer characteristics of tetra-n-butyl ammonium bromide clathrate hydrate slurry during flow melting and generating in a double-tube heat exchanger Z.W. Ma, P. Zhang ⇑ Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China article info Article history: Received 7 September 2011 Received in revised form 15 June 2012 Accepted 20 June 2012 Available online 28 June 2012 Keywords: Heat exchanger Pressure drop Heat transfer Slurry Non-Newtonian fluid abstract This study presents the flow and heat transfer characteristics of tetra-n-butyl ammonium bromide (TBAB) aqueous solution and clathrate hydrate slurry (CHS) in a double-tube heat exchanger (DHE). Both the TBAB aqueous solutions at the supercooling state and normal state were tested to understand the influence of supercooling state on the flow and heat transfer of TBAB aqueous solution. Pressure drops of 5–25 wt% TBAB CHS flowing through DHE without heat exchange were measured and the correspond- ing flow friction factor was obtained, while the pressure drops of the flow melting and generating of TBAB CHS were also obtained. Single side heat transfer coefficients of the flow melting of 10–25 wt% TBAB CHS in the heat exchanger were presented, and the local heat transfer correlation was developed with the consideration of the effect of latent heat on the heat transfer. Moreover, the heat transfer during TBAB CHS generation process was found to be deteriorated by the adherence of crystals on the heat transfer surface, and the heat transfer improvement by the melting operation was shown. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The traditional refrigerants, such as CFCs and HCFCs, have high ozone depletion potential and global warming potential. Therefore, the phase-out of these environment-negative-impact refrigerants is one of the important issues all over the world. On the other side, it is estimated that approximately 30–40% of the total energy in a building is consumed by the air conditioning system in summer time, therefore improving the energy efficiency of the air condi- tioning system is quite necessary for energy conservation. The uti- lization of phase change slurry in air conditioning system as cold storage and transport medium can achieve both the environment protection and energy conservation. The phase change slurry can behave as the secondary refrigerant in air conditioning system, which reduces the charge amount of primary refrigerant and there- fore the leakage of primary refrigerant to the environment. Com- pared to the single-phase secondary refrigerant, e.g. chilled water, the cold carrying capacity of phase change slurry is larger due to the involved latent heat in the melting process, therefore the pumping power for the fluid circulation is reduced since low flow rate is required. The cold storage implemented by the phase change slurry can shift the peak-load of the air conditioning to the off-peak times, which reduces the scale of the system, and con- sequently improves the energy efficiency. Ice slurry, emulsion and microencapsulated phase change slurry are the most commonly used phase change slurries. However, drawbacks exist in their applications. Ice slurry requires low gen- eration temperature, therefore the energy efficiency of the refriger- ator is low. Phase change particles in emulsion are easy to stick to each other, causing the block in pipe. Microencapsulated phase change slurry has relative low thermal conductivity and the long-term operation will deteriorate its performance. Therefore, it is necessary to look for a new phase change slurry for air conditioning. Recently, tetra-n-butyl ammonium bromide ([CH 3 (CH 2 ) 3 ] 4 NBr, TBAB) clathrate hydrate slurry (CHS) was investigated world- widely and was considered as a promising candidate of phase change slurries used in air conditioning system [1–4]. TBAB in water can form two types of hydrates with different hydration numbers at the atmospheric pressure condition, named as type A and type B. It was reported that the melting temperature of TBAB CHS was in the range of 0–12 °C depending on its initial aqueous solution concentration, which was appropriate to the air condition- ing application. The cold carrying capacity of TBAB CHS was large due to the involved latent heat of hydrate crystal in the melting process, which was about 2–4 times as large as that of chilled water at temperature range of 5–12 °C. Due to the small size of hy- drate crystals, the fluidity of TBAB CHS was also good if the solid fraction was not very high. TBAB CHS was normally treated as non-Newtonian fluid due to its solid-liquid two-phase feature. Darbouret et al. [5] and Song 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.06.012 ⇑ Corresponding author. Tel.: +86 21 34205505; fax: +86 21 34206814. E-mail address: zhangp@sjtu.edu.cn (P. Zhang). Experimental Thermal and Fluid Science 44 (2013) 227–234 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs