Experimental and CFD estimation of heat transfer in helically coiled heat exchangers

Abstract

Enhancement in heat transfer due to helical coils has been reported by many researchers. While the heat transfer characteristics of double pipe helical heat exchangers are available in the literature, there exists no published experimental or theoretical analysis of a helically coiled heat exchanger considering fluid-to-fluid heat transfer, which is the subject of this work. After validating the methodology of CFD analysis of a heat exchanger, the effect of considering the actual fluid properties instead of a constant value is established. Heat transfer characteristics inside a helical coil for various boundary conditions are compared. It is found that the specification of a constant temperature or constant heat flux boundary condition for an actual heat exchanger does not yield proper modelling. Hence, the heat exchanger is analysed considering conjugate heat transfer and temperature dependent properties of heat transport media. An experimental setup is fabricated for the estimation of the heat transfer characteristics. The experimental results are compared with the CFD calculation results using the CFD package FLUENT 6.2. Based on the experimental results a correlation is developed to calculate the inner heat transfer coefficient of the helical coil.

Introduction

It has been widely reported in literature that heat transfer rates in helical coils are higher as compared to a straight tube. Due to the compact structure and high heat transfer coefficient, helical coil heat exchangers are widely used in industrial applications such as power generation, nuclear industry, process plants, heat recovery systems, refrigeration, food industry, etc. (Xin et al., 1996, Abdulla, 1994, Bai et al., 1999, Jensen and Bergles, 1981, Futagami and Aoyama, 1988, Patankar et al., 1974).

Heat exchanger with helical coils is used for residual heat removal systems in islanded or barge mounted nuclear reactor system, wherein nuclear energy is utilised for desalination of seawater (Manna et al., 1996). The performance of the residual heat removal system, which uses a helically coiled heat exchanger, for various process parameters was investigated by Jayakumar and Grover (1997). The work had been extended to find out the stability of operation of such a system when the barge on which it is mounted is moving (Jayakumar et al., 2002). In all these studies, empirical correlations were used to estimate the amount of heat transfer and pressure drop in the helical coils. The appropriateness of the correlation used in the above work is uncertain for the specific application and in the present work, it is proposed to generate the desired correlations using numerical and experimental work.

Heat transfer and flow through a curved tube is comprehensively reviewed first by Berger et al. (1983) and subsequently by Shah and Joshi (1987). The latest review of flow and heat transfer characteristics is provided by Naphon and Wongwises (2006). The characteristics of flow, pressure drop and heat transfer have been reported by many investigators. The heat transfer enhancement in helical coil systems is reported by Prabhanjan et al. (2004), Berger et al. (1983), Janssen and Hoogendoorn (1978) and Ruthven (1971). Condensing heat transfer and pressure drop of refrigerant R 134A in helicoidal (helical double pipe heat exchanger) is experimentally investigated by Kang et al. (2000). The effect of torsion on the flow in a helical tube of circular cross-section is experimentally investigated by Yamamoto et al. (1995) for a range of Reynolds numbers from about 500 to 20,000.

Most of the investigations on heat transfer coefficients are for simplified boundary conditions such as constant wall temperature or constant heat flux (Prabhanjan et al., 2004, Shah and Joshi, 1987, Nandakumar and Masliyah, 1982). The situation of constant wall temperature is idealised in heat exchangers with phase change such as condensers. The boundary condition of constant heat flux finds application in electrically heated tubes and nuclear fuel elements. However, the case of fluid–fluid heat exchange has not been studied well. Experimental studies of a double pipe helical heat exchanger was conducted by Rennie and Raghavan (2005). The double pipe helical coil heat exchanger was further numerically investigated by Rennie and Raghavan, 2006a, Rennie and Raghavan, 2006b. Pressure drop and heat transfer in tube-in-tube helical heat exchanger was studied by Kumar et al. (2006). For a double pipe heat exchanger, the co-current or counter-current flow situation can be applied. However, in the helically coiled heat exchanger, which is taken up in the present study, practically cross flow exists in the shell side and hence the analysis is entirely different from those reported in earlier studies.

In this work, it is proposed to generate correlations for inner heat transfer coefficient considering fluid–fluid heat exchange in a helically coiled heat exchanger. An experimental setup is built for carrying out the heat transfer studies representing the equipment under study. For further details of the equipment and its applications, refer to Jayakumar and Grover (1997). In addition, the heat transfer phenomena in the exchanger is analysed numerically using a commercial CFD code FLUENT Version 6.2 (2004). In contrast to the earlier similar analyses, instead of specifying an arbitrary boundary condition, heat transfer from hot fluid to cold fluid is modelled by considering both inside and outside convective heat transfer and wall conduction. In these analyses, we have used temperature dependent values of thermal and transport properties of the heat transfer medium, which is also not reported earlier. The numerical predictions are verified against the experimental results.

The article is organised as follows: we begin with the introduction of helically coiled system followed by the description of the experimental setup used for heat transfer studies and the methodology of experimentation. In the next section, the numerical results for heat transfer characteristics of a helical pipe are presented. Subsequently the actual heat exchanger is analysed for the case of fluid–fluid heat transfer and the computational results are compared with the experimental ones. In the last section, the validated results are converted into a correlation for the estimation of heat transfer inside a helical coil as appropriate to the experimental conditions.