Modelling of Air Movements in Rooms

The assessment of room air movements, in all but elementary cases, relies on investigations using either full-size mock-ups or scaled models. Temperature considerations severly limit the maximum geometric scale factor. A solution is offered by which accurate predictions of the air flows in full-size air-conditioned rooms may be obtained from observations. made with small models if certain criteria are satisfied. The maximum geometric scale-factor can be increased to 8.5, while limiting the maximum working temperature in the model to 100°C, by replacing the convective currents with wall jets of a similar velocity profile, volume flow, momentumfl ux and heat content. A further improvement may be achieved if the scale-factor adopted for the jet nozzle is smaller than the geometric scale-factor. This approach can lead to scale-factors exceeding 11.8. Theoretical studies have shown the replacement of convective currents by plane jets is feasible. In the course of the study, detailed investigations of areas important for the aimsof the project but where there is a dearth of relevant information, were undertaken. To test the validity of predictions and to establish necessary empirical factors, a range of measurements of convective currents and their replacement jets were'carried out. The results showed that a virtual identity of maximump rofile velocity, momentumfl ux and volume flow at the replacement cross-section could be achieved. Based on measured empirical factors, a simple procedure, valid for the majority of practical applications, by which replacement jets can be calculated directly from convective surface parameters is given. Thus, the aim of this study, namely the worthwhile use of a small model which can be constructed cheaply, has been achieved

Rate of heat recovery from a hot-water store: Influence of the aspect ratio of a vertical-axis open-ended cylinder beneath a submerged heat-exchanger

A coiled finned-pipe heat-exchanger was employed to extract heat rapidly, from a 90-litre hot-water charged tank; the water being initially at a temperature of approximately 80°C. Free-convective buoyancy movements of the water around the outside of this coiled pipe (immersed in the store) occur as a result of initially-cold water (at 20°C) being forced internally through the heat-exchanger's pipe. The axis of the heat-exchanger coil is orientated vertically, and the heat-exchanger's inlet is arranged to be at its lowest level. The influence of a smooth, vertical-axis cylindrical PVC baffle located symmetrically beneath the heat-exchanger upon the rate of heat recovery via the heat-exchanger was investigated. Irrespective of the flow rate of water through the heat-exchanger's pipe, the greatest rate of heat recovery was achieved using the baffle index (a function of the height, diameter and depth of the baffle within the thermal store as well as the latter dimensions) equals 1·70±0·05. The improvement in the heat-exchanger's effectiveness is approximately 2·5 to 3%. Increasing the rate of water flow through the heat-exchanger's pipe resulted in a reduction of the heat-exchanger's quality-effectiveness, but a rise in the thermal store's recuperation effectiveness. The latter was due to the reduced amount of mixing between the ascending warm-streams and the descending colder-streams of water in the tank. The experimental, two-dimensional radial temperature-distribution can be employed to predict (to ±5·0%) the cumulative amount of heat recovered from the thermal store.

Thermal-energy stores for supplying domestic hot-water and space-heating

Factors influencing the ability of a conventional domestic hot-water tank to deliver domestic hot water rapidly have been culled, collated or assessed experimentally. It has been deduced that, with a horizontal-axis coiled, finned-type, heat exchanger--immersed in the traditional domestic hot-water tank--for extracting heat from the tank where and Ra are the mean Nusselt and Rayleigh numbers, respectively, for the freely-convecting flow in the tank. Among the design conclusions are: 1. (i) The optimal value of the ratio of height-to-diameter for the thermal store lies between 3 and 4. This is a compromise between achieving the improvements associated with (a) having a high degree of stratification (e.g. resulting from the use of a tall tank) and so facilitating rapid heat removal, and (b) minimising the surface area (and hence the rate of wild heat loss from the store). 2. (ii) Preferably, the walls of the thermal store should be made of a low-thermal-conductivity material and, provided mechanical integrity can be assured, only be of small thickness, thereby enhancing the degree of stratification achieved in the store. 3. (iii) An approximate value for the optimal thickness of thermal insulant applied to the tank, in order to minimise the wild heat losses through the tank's walls, can be calculated. Thus, for a typical insulant, it is recommended that a thickness of more than three times that traditionally used on domestic hot-water tanks be applied. 4. (iv) The use of a horizontal plate, located near the middle of the store, can lead to small increases in the rate of useful heat recovery from the store.

The performance of a coiled finned-tube heat-exchanger submerged in a hot-water store: The effect of the exchanger's orientation

A coiled-pipe exchanger was employed to extract heat rapidly at relatively high temperatures from a 90-litre hot-water charged tank, the water being initially at a temperature of approximately 80°C. The free-convective movements of the water around the outside of the coiled pipe (immersed in the store) were due to buoyancy forces induced by colder water being forced through the heat-exchanger's pipe. For the heat-exchanger orientations tested, the maximum effectiveness, with respect to the quality of the heat extracted was achieved (i) by having the axis of the coiled heat-exchanger arranged horizontally with its inlet at the lowest level; and (ii) with the lower rate tested (=6·6 litre/min) of water being passed through the heat-exchanger's pipe, partly because this led to a lower rate of disruption of the stratification of the water within the store.

Influences of baffles on the rate of heat recovery via a finned-tube heat-exchanger immersed in a hot-water store

The heat transfer at the external wall of the heat exchanger occurs primarily by buoyancy-driven natural convection in the surrounding water. An analysis of the effects of the presence of a rectangular duct on this heat-transfer process is presented. Small improvements in the rate of heat recovery were obtained repeatedly when a horizontal plate was located in the middle of the store.

Heat-transfer correlations for an immersed finned heat-exchanger coil transferring heat from a hot-water store

Natural-convection heat transfers, to a finned-tube heat-exchanger coil immersed in a hot-water store, have been investigated. Cold water was passed through the pipe of the heat-exchanger in order to extract heat rapidly from the hot water in the store. Natural convection currents in the stored water were created by buoyancy forces, which were induced by the temperature gradients that developed as a result of the heat-extraction process. A heat-transfer correlation in terms of Nusselt and Rayleigh numbers has been deduced in order to predict the natural convection heat-transfer coefficient on the outside surface of the heat exchanger. This correlation, which is valid for heat entering the fins, to within an accuracy of better than 4%, is: Nu=0·280 Ra0·293 for 100

Free-convective flows within a hot-water store, induced by a submerged, relatively cold heat exchanger

A coiled finned-pipe heat exchanger was employed to extract heat rapidly, at temperatures in excess of 30° C, from a 90-litre hot-water charged tank; the water being initially at a temperature of approximately 80° C. Free-convective buoyancy movements of the water around the outside of this coiled pipe (immersed in the store) occur as a result of initially-cold water (at ~ 20° C) being forced internally through the heat exchanger's pipe. For the most rapid rate of heat extraction, the axis of the heat exchanger coil should be oriented horizontally, and the heat exchanger's inlet arranged to be at the lowest level. Increasing the rate of water being passed through the heat exchanger's pipe resulted in a reduction of the heat exchanger's quality effectiveness, but a rise in the thermal store's recuperation effectiveness. The latter was due to the reduced level of mixing between the ascending warm streams and the descending colder streams. The development of the thermal boundary layer adjacent to the tank's wall was beneficial in deflecting the centrally descending stream of relatively colder water (which comes off the heat exchanger's pipe) to drive the ascending warmer water onto the heat exchanger, which is located near the surface of the water in the tank.

Performances of modern domestic hot-water stores

Several designs of domestic hot-water (DHW) store, including those with immersed heat-exchangers (HXs), are commercially available. So there is a need for a method that accurately assesses their effectivenesses. In this study, the behaviours of a novel stratified, and two standard, stores were analyzed. The TRNSYS simulation software was enhanced to simulate the functioning of those stores. The resulting mathematical model was validated using measurements obtained from experiments, which required a realistic daily DHW draw-off for testing the DHW systems. Evaluation of a user-related effectivenesses (URE) for each of the three tanks tested showed that the inner configurations of: (i) the tank and (ii) the immersed HX can significantly affect the store's performance. The stratified store was up to 32% more effective than the commonly employed commercially-available store.DHW store Immersed heat-exchanger Store' s performance Experimental analysis