M.J. Shilston

Heat exchanger: Optimal separation for vertical rectangular fins protruding from a vertical rectangular base

An experimental investigation of the steady-state rates of heat transfer from an array of vertical rectangular 3 mm thick fins, extending 60 mm perpendicularly out of a 250 mm high vertical rectangular base, is reported. For base temperatures between room temperature (~ 15°C) and 100°C, the optimal separation of the parallel fins, corresponding to the maximum rate of heat loss, is 10 ± 1 mm.

Heat exchanger design: Thermal performances of rectangular fins protruding from vertical or horizontal rectangular bases

An experimental investigation of the steady-state rates of heat transfer from an array of vertical rectangular fins of 3 mm thickness and 250 mm length, protruding 60 mm perpendicularly upwards from a 250 mm x 190 mm horizontal rectangular base, is reported. For constant (to ±0·1°C) base temperatures between 40°C and 80°C, in an ambient environment of 20±0·2°C, the optimal separation of the parallel fins, corresponding to the maximum rate of heat loss, is 10·5±1·0 mm. The effects of the extent of the fin protrusions on the thermal performances of such vertical fins, on the same base, which was arranged to be either vertical or horizontal, have been studied. The experiments were performed with three different fin protrusions, namely 32 mm, 60 mm and 90 mm, for a base temperature of 40°C above that of the ambient environment. The steady-state rate of heat dissipation from the fin array increased slightly less than linearly with the fin protrusion for both orientations, but the relationship became closer to linear as the fin spacing was increased. A comparison of the abilities to dissipate heat to the room air from the same geometrical configuration having a rectangular fin array but positioned with vertical fins on a vertical base, vertical fins protruding upwards from a horizontal base, or horizontal fins on a vertical base, has been made. The orientation with vertical fins protruding upwards from the horizontal base, is the preferred option because of the relatively high rates of heat transfer that can then be achieved.

Forced-convective steady-state heat transfers from shrouded vertical fin arrays, aligned parallel to an undisturbed air-stream

An experimental investigation of the steady-state rates of heat loss from an array of vertical rectangular fins of 3 mm thickness and 250 mm length, when in the presence of an almost adiabatic horizontal shroud, situated adjacent to and above the horizontal fin tips, is reported. With the fins horizontal base at a uniform temperature of 40 ± 0·1° C above that of the ambient environment, the optimal fin separation--corresponding to the maximum rate of heat loss--is deduced. As the ratio of the shroud height above the vertical fins to the fin height decreases from unity to zero, this optimal value decreases by approximately 17%. The frictional characteristics of the air flow through the fin array have also been studied in the Reynolds number range of 4·0 x 104 to 2·0 x 105. Large streamwise pressure drops and high heat-transfer rates resulted when the fins were closely spaced and no clearance gap was present above the vertical fins. In reasing the shroud clearance resulted in smaller overall pressure drops and decreasing heat-transfer rates from the heat exchanger.

Heat transfer performances of vertical rectangular fins protruding from rectangular bases: Effect of fin length

The effects of increasing the fin length from 250 to 375 mm on (i) the steady-state rate of heat loss and (ii) the optimal uniform fin separation of vertical rectangular fins protruding from a horizontal or a vertical rectangular base, have been investigated experimentally. A constant base temperature, 40 (±0·3)°C above that of the ambient environment, was used.

Heat exchanger design: Optimal uniform separation between rectangular fins protruding from a vertical rectangular base

Steady-state rates of heat loss, from an array of 3 mm thick, 250 mm long, horizontal rectangular duralumin fins extending 60 mm perpendicularly out of a 250 mm x 190 mm vertical rectangular duralumin base, have been measured. With the base, at a uniform temperature of between 40°C and 80°C, in a 20°C ambient environment, two separations of the parallel fins, corresponding to maxima in the rate of heat loss versus fin separation curves ensued, respectively at 12 ± 1 mm and 38 ± 1 mm. The use of the latter maxima (i.e. the optimal separation) leads to the higher rate of heat loss. The heat transfer performances of vertical and horizontal rectangular fin arrays on a vertical rectangular base are compared: using the same geometrical configuration and identical base temperatures in both cases, the vertical fin orientation has the more rapid, steady-state heat loss.

The total energy approach: Evolution of combined heat and power for district heating and/or cooling

Combined heat and power (CHP) generation is not a new concept, but it provides an elegant solution to some of our present fuel problems, offering, as it does, 80% or greater efficiency. However, Great Britain lags behind other European countries with respect to the rate of introduction of CHP together with district heating and/or cooling (DHC) systems. Reviews of (i) the historical development of the designs of DHC pipelines, from insulated pipes in air-filled ducts to the modern preinsulated pipes buried in the ground, and (ii) sources of energy as well as developments in metering and control, for CHP-DHC systems, are presented. The cost effectiveness of each CHP-DHC system is highly sensitive to unit fuel prices, current discount rate, as well as the capital cost incurred. In the best interests of Britain, major governmental investments are now needed urgently in order to encourage the wider adoption of these systems.

Natural convection across cavities: Design advice

Experimental measurements and theoretical predictions of steady-state heat transfers to or from horizontal single or double pipelines enclosed in horizontal circular or rectangular enclosures have been collated. The optimal configurations of the pipelines to achieve maximum thermal resistances of the air-filled cavities are identified. A recommended correlation for predicting the combined convective/conductive resistances provided by the contained air in a horizontal concentric annuli is presented.

Optimal eccentric annuli (Containing atmospheric-pressure air) for thermally insulating, horizontal, relatively cold pipes

The geometrical configurations of those air-filled, horizontal eccentric annuli, that provide maximum thermal insulation corresponding to various imposed temperature conditions have been identified. For the concentric system, a simple correlation is presented which will permit the calculation of the associated steady-state rates of convective/conductive heat leak through the air annulus to the horizontal pipe conveying a chilled fluid. The conclusions and recommendations should be of use to designers, especially of district-cooling networks.

Thermal insulation provided by plain, horizontal annular cavities containing atmospheric pressure air

Abstract For the range 3 × 103 ≤ Grdi ≤ 108 and 1·3 ≤ r 0 r i ≤ 7·5 , it is suggested that Nu δ = 0·181( r 0 r i ) − 0·215 Gr d i 0·25 for the steady-state rate of heat transfer outwards by combined laminar, free convection and conduction through the atmospheric pressure air contained within horizontal concentric annuli. This simple correlation, evolved from an analysis of published, as well as new, experimental information, will enable designers to predict the combined convective/conductive resistance provided by the contained air for the range of concentric pipes likely to be encountered in practice. An optimal eccentricity of 0·24 (the inner cylinder being moved vertically upwards relative to the outer cylinder from the concentric position) corresponds to the maximum combined convective/conductive resistance configuration. For the systems tested in the temperature range 18°C ≤ T ≤ 150°C, this optimal eccentricity is not significantly affected by changes in the surface emissivities.

Steady State Heat Losses from Horizontal Pipes in an Air-Filled Rectangular Concrete Duct

Factors influencing the steady state heat loss behaviours of horizontal ‘supply’ and ‘return’ hot-water pipes, within an atmospheric pressure air-filled, relatively cold, horizontal rectangular trench, are considered. An experimental investigation concerning the effects of the displacement ratios for the two pipes revealed the optimal configuration, that is one which achieves a minimum steady state rate of heat loss from the supply pipe. For one set of temperatures for the pipes and trench walls, the optimal configuration of the supply and return pipes occurred at displacement ratios of +0.70 and −0.05, that is with the supply pipe in the upper region of the cavity and the return pipe vertically below it, the pipes being equidistant from the vertical walls of the trench. This configuration is of significance with respect to achieving maximum energy thrift for district heating pipelines, because it differs radically from the ‘side-by-side’ arrangement of pipes conventionally adopted in district heating practice.