Heat transfers from district heating pipes.

Experimental and numerical investigations were carried out on air-filled cavities containing heated inner cylinders. The effect of varying the position of radial spacers on a single cylinder was studied. It was concluded that for central positioning of the cylinder within the cavity. the rate of heat-transfer was minimised at a radial spacer angle of 480 (measured from the vertically downwards radius vector). When the cylinder was positioned at displacement ratio of 0.7, the rate of heat-transfer was minimised at a corresponding spacer angle of 520. The corresponding reductions in the total rate of heat-transfer were found to be 25% and 31% less than that obtained for the system with no spacers at a cylinder displacement ratio of zero. Following this research investigation, the behaviour of a two-pipe arrangement, consisting of a hot supply and cooler return pipe within a rectangular sectioned cavity, was studied. Eccentric positioning of both supply and return pipes showed that minimum rates of heat-transfer occur at supply and return pipe displacement ratios of 0.45 and -0.33 respectively. This value of heat-transfer is approximately 20% less than that obtained for a system where supply and return pipe displacement ratios are 0.7 and zero respectively. As experimental testing has proved to be excessively time consuming (e. g. due to having to wait until a steady-state ensued before measurments were taken) and laborious, a finite-element numerical model was developed and used to predict the heat-transfer between a heated inner cylinder and a cooled outer square duct. This study investigated eccentricity effects on the rate of heat-transfer for different ratios of duct height to cylinder radius. Solutions were obtained for Rayleigh numbers 1 to 300 and optimal pipe eccentricity for minimum heat-transfer was predicted. These predictions were in good agreement with previous experimental results