A novel method of improving the thermal efficiency of heat exchangers by altering the internal profile of the tubes can double the rate of heat exchange through the tube wall, say researchers at Manchester’s UMIST laboratories. The method involves forming specially profiled spiral flutes on the inner bore of the tubes.
For many years it has been known that roughening the bore of heat exchanger tubes influences the heat exchange rate. The problem has been that the extra friction caused by the rough surfaces increases the pressure drop across the heat exchanger. Also the rough surface can also cause unwanted clogging of the tubes if there are particles in the heat exchange medium.
How can the benefit of improved heat exchange be gained without sacrificing the flow rate or pushing up the pressure drop? That was the challenge facing research engineer, Dr Tim Craft at UMIST. The answer lay in using computational fluid dynamics (CFD) to analyse exactly what happens to flow and heat transfer inside a heat exchanger tube in order to optimise the design of the tube.
The development is based on an earlier idea which came from the USA in the 1970s. This proposed that the bore of heat exchanger tubes should consist entirely of vee-shaped grooves, set in a spiral around the inner wall of the tube. The grooves were to be symmetrical about the tube’s radius, but the idea was never put into practice.
An advantage of the simple uniform flutes was that, unlike simply roughening the bore to improve heat transfer, they did not increase the drag of the fluid on the wall. Thus there was no increase in the pressure drop across the heat exchanger.
UMIST engineers have taken the idea a stage forward, using computational fluid dynamics to study what happens to the flow inside the flutes. They discovered that a symmetrical vee-shaped flute was not the ideal profile for optimum heat transfer. Analysis of the dynamics of the fluid flow that a modified, non-symmetrical groove could offer dramatic improvements.
The optimum non-symmetrical flute profile has a steeper slope on the leading face and a flatter slope on the trailing face. It also has a narrower angle, 25 degrees instead of 30 degrees proposed in the original idea. When this is added to forming the grooves in a spiral of some 25 degrees to the axis of the tube, swirl velocities are induced in the heat transfer fluid flowing in the grooves as it travels along the length of the tube.
This imparts a secondary motion – or velocity vector – inside the flutes which circulates and `scours’ the wall of the tube, removing the cooler fluid from the wall, continuously replacing it with fresh hot fluid from the main stream.
The result is to maintain a higher average wall temperature which increases the thermal gradient across the wall, and results in a higher heat transfer coefficient. The beneficial effect of reducing the drag of the fluid on the tube wall, and hence the pressure drop across the heat exchanger, is retained.
The results of the computational study have been confirmed by measurements in a 125mm bore tube in a test rig.
Two UK manufacturing companies are currently examining the concept of the heat exchanger with UMIST who have applied for worldwide patents.