REVIEW ON CFD ANALYSIS OF DOUBLE TUBE HELICAL COIL HEAT EXCHANGER

Vikas Singh 1 , Subhashini Verma 1 , Priyanshu Dradhomar 1 , Pallavi Dradhomar 1 and Manjunatha. M 2 . 1. Department of Mechanical Engineering, JSS Academy of Technical Education, Noida, India. 2. Assistant Professor, Department of Mechanical Engineering, JSS Academy of Technical Education, Noida, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

This literature review focuses on the study and applications of Computational Fluid Dynamics (CFD) in the field of heat exchangers. It has been observed cfd analysis is a very useful tool in the field of heat exchangers to determine the heat transfer characteristics. Different models available in general purpose commercial CFD tools i.e. standard, realizable, energy model, radiation and k-ε (2eqn.), with velocity-pressure coupling schemes such as SIMPLEC have been adopted to carry out the simulations. The qualities of the solutions obtained from these simulations arewithin the acceptable range with very accurate results. Hence proving CFD as an effective tool for predicting the behavior and performance of a wide variety of heat exchangers.

…………………………………………………………………………………………………….... Introduction:-
Heat exchangers play an important role in the operation of systems such as process industries, power plants and heat recovery units. Its inevitable need has necessitated work on efficient and reliable designs leading towards optimum share in the overall system performance. The Log Mean Temperature Difference (LMTD) and the number of heat transfer units (NTU) method are two methods used for design of heat exchanger. These methods have some limitations i.e. iterative in nature and need of a prototype to implement the design. Due to these reasons, these methods are time consuming as well as uneconomical especially for large scale models. However, economical access to powerful microprocessors has created a way for the evaluation of Computational Fluid Dynamics (CFD). CFD is a science that can help to study fluid flow, heat transfer, chemical reactions etc. by solving mathematical equations using numerical analysis. It is also very helpful in designing a heat exchanger system from scratch and in troubleshooting or optimization of the design. CFD employs a very simple principle of resolving the entire system into small cells or grids and applying governing equations on these discrete elements to find numerical solutions regarding pressure distribution, temperature gradients and flow parameters in a shorter time and at a lower cost, as the required experimental work is reduced.
The results obtained with the CFD are of acceptable quality. In the current work, various parameters changes have been done in the design of heat exchangers and their solutions with the help of CFD have been reviewed. This work can serve as a ready reference for application of CFD in design of various types of heat exchangers.

Int. J. Adv. Res. 5(4), 1758-1762
1759 Types of heat exchangers:-There are innumerable types of heat exchangers in use. It is not limited to plate type heat exchanger, shell-and-tube heat exchanger, etc. but broad classification of heat exchangers based on their construction is outlined in the Fig. 1 below. The double tube helical coil heat exchanger is given in fig.2. The heat exchanger in the figure is a 2 turn heat exchanger. 1760 Literature Review:-D. G. Prabhanjanet al. found that use of a helical coil heat exchanger result in increase of heat transfer coefficient compared to a similarly dimensioned straight tube heat exchanger. They also observed that both heat exchangers had higher heat transfer coefficients when the bath temperature was increased, may be due to increased buoyancy effects. Flaw rate did not affect the heat transfer coefficient; due to the fact that the flow was turbulent and increasing the flow rate does not greatly change the wall effects. Temperature rise of the fluid was found to be effected by coil geometry and by the flow rate.
J.S. Jayakumara, et al. observed the following Characteristics of heat transfer under turbulent flow of singlephasewater through helical coil heat exchanger.
Analysis was carried out for both the constant wall temperature and constant wall heat flux boundary conditions. Fluid particles were found to undergo oscillatory motion inside the pipe and this caused fluctuations in heat transfer rate.
Nusselt number on the outer side of the coil i.e. far from coil axis wasfound to be highest among all other points at a specified cross-section, while that at the inner side of the coil is the lowest.
A number of numerical experiments were carried out to study influence of coil parameters, viz., pitch circle diameter, coil pitch and pipe diameteronheat transfer characteristics. The coil pitch was found to behaving significance only in the developing section of heat transfer. To analyze the effect of tube diameter on the heat transfer coefficients, the heat transfer coefficients were evaluated for the different tube diameter keeping inner coil pitch circle diameter, outer coil pitch circle diameter and velocity constant. These results shows that the heat transfer coefficient increases with the increase in tube diameter. This result is quite interesting and does not agree with the expected flow behaviour. This is primarily due to reduction in coil gap with increasing tube diameter and the effect of tube diameter is not dissociated with the effect of coil gap. The effect of coil gap was found to be dominating over the effect of the tube diameter.
J.S. Jayakumaraet al.validatedheat transfer calculations for the two-phase flow against experimental results of flow through an annular pipe. Studies were carried out by varying (i) coil pitch, (ii) pipe diameter (iii) pitch circle diameter and (iv) void fraction at the inlet. Unlike the flow through a straight pipe, the centrifugal force caused due to the curvature of the pipe causes heavier fluid (water phase) to flow along the outer side of the pipe. Hence high velocity and high temperature were observed along the outer side. They observed that torsion caused by pitch of the coil madethe flow unsymmetrical about the horizontal plane of coil. As the pitch was increased, higher velocity and higher temperature regions were observed on the bottom half of the pipe. They also found that increase in pipe diameter causes higher heat transfer coefficient and lower pressure dropkeeping the inlet velocity constant. This effect is due to the influence of secondary flows. As the pitch circle diameter was increased, the centrifugal forces decreases and this causes reduction of heat transfer coefficient anddrop in pressure. Conclusions:-1. By reviewing different journals it was observed that the double tube helical coil heat exchanger has greater heat transfer rate as compared to straight double tube heat exchanger. 2. It requires less space for same temperature dropi.e. it is compact. This is highly useful as we can use in plants which have less area. 3. Due to centrifugal action high velocity and high temperature is observed outside. 4. CFD is found to be very useful tool for the analysis of double tube helical coil heat exchanger as results obtained from CFD analysis were withinclose tolerance with practical values. 5. It was observed that epsilon 2-eqn model gives most accurate result for this heat exchanger.