ABSTRACT
Compact tube heat exchanger is need of the day. Several designs are tried and tested by the thermal engineers in order to augment heat transfer. For the assessment of thermal performance of any novel design a prototype is prepared and tested under different conditions. This paper discusses the thermal performance evaluation of the spiral tube heat exchanger (SHEx) considering only single side of the tube in to analysis. In order to achieve steady state thermal condition heat exchanger is immersed in to a reservoir of cold water. Quantity of such water is decided based on the rate of increase in the temperature of the reservoir, which is observed approximately up to 1 Deg/min. Based on this assumption of quasi-steady condition (or quasi-semi-infinite lump of the thermal mass), analysis is carried out. Rigorous experiments are conducted on such a set-up, for various mass flow rates of water inside the tube with varying heat loads. Such experiments are considered for both the types of fluid flow configuration, i.e. inward flow as well as for outward flow conditions. Results after analysis are summarized as Eq. (1) and Eq. (2) for inward and for outward flow configuration respectively, for NU = f(Rea Pr0.3) type relationship, as under.
Nu = 9.345 * 10-4 (Re) 0.8795 (Pr) 0.3 (1)
Nu = 1.586 * 10-4 (Re) 1.0674 (Pr) 0.3 (2)
From above it is observed that if design called for only one spiral path (e.g. cross flow spiral heat exchanger) in that case preferred path of the fluid should be outward flow configuration. However if design called for both spirals paths side by side, in that case both should be provided in counter-flow fashion and mass flow of incoming fluid should be kept as determined by following condition, up to the drop in water temperature of 15°C.
mi=0.140153diμi(m0/d0μi)1.2136 (3)
Keywords: Compact heat exchanger, Spiral Tube Heat Exchanger, Outward flow, Reynolds number, Semi-infinite medium.
