Experimental research on the enhancement 96&H 563 boiling heat transfer of liquid helium In narrow channel Guo. T. and Zhu, T. Cryogenics, 1997, 37, (2). 67-70.
Boiling is produced from a heated surface immersed in liquid, in the nucleate pool. Bubble growth comes mainly from the evaporation of a liquid microlayer between the bubble and the heated surface below. If boiling is confined in a vertical narrow channel with a small gap, the bubble size will be restricted between surfaces. A growing vapour bubble from the surface will soon touch the neighbouring surface, be forced to take a flat shape and leave its nucleation site earlier than in a pool boiling. This results in a higher boiling heat transfer rate. On the other hand, rising vapour bubbles will collide with those bubbles still attached to the surface and bring them away before they can leave the surface by themselves, further enhancing boiling heat transfer. The experiment showed that boiling in a narrow channel reduces the temperature difference between liquid helium and the heated surface. Boiling heat transfer rate (or heat flux) in a narrow channel is also much higher than in a pool boiling. A new nucleate boiling heat transfer enhancement method is presented. The method could be useful in designing superconducting a.c. magnets that are bath cooled by liquid helium.
Hydronic heating systems: transient 96lOl564 validation and load matching control
New alternatlves for R22
Zhao, Y. er al. Energy, 1997, 22, (7), 669-673. Analysis of the R22 alternatives R32/125/152a and R290 and their mixtures took place in a room air conditioner. The cooling capacities, energy efficiencies, compressor-discharge temperatures, and pressures of alternatives have been compared with R22. R321 125/152a mixtures were found to be the most effective coolants.
Numerical simulation of a latent heat thermal energy storage system with enhanced heat conduction
Costa, M. et al. Energy Cowers. Mgmt, 1998, 39, (3/4), 295-301. In order to take advantage of the off-peak electrical energy for space heating, a latent heat storage system has been designed. The thermal performance of such a storage system with and without fins has been analysed using an enthalpy formation and a fully implicit finite difference method. Calculations have been made for the melt fraction and energy stored for conduction plus convection and the conduction modes of heat transfer for the one-dimensional simulation model. For the two-dimensional model, the same calculations have been made for conduction only. The magnitude of the melt fraction with fins is found to be considerable, dominating the melt fraction when no fin is used.
Zaheer-Uddin, M. and Monastiriakos, P. Inr. J. Energy Res., 1998, 22, (1). 33-46. The authors present the development of a dynamic model of a hydronic heating system. It consists of a boiler, baseboard terminal units, domestic hot water (DHW) heat exchanger coil and an environmental zone. The model is described by a set of time varying non-linear coupled differential equations. Predicted responses from the model are compared with the measured data gathered on an on-off controlled hydronic heating system installed in an apartment building. Results show that the model predictions compare well with the field data. Using this validated model, feedback controllers are designed to achieve better regulation of zone air temperature, boiler water temperature and DHW temperature. A load tracking setpoint control strategy is proposed to regulate boiler temperature as a function of outdoor air temperature. Results showing the simulated responses of the system with the designed controllers subject to step changes in space heating and DHW loads are given.
Impact of cool-storage air-conditioning on power96lOl565 system planning in Thailand Surapong, C. and Bundit, L. Energv, 1997, 22, (8) 743-749. The paper assesses the impact of the application of cool-storage airconditioning on system load shape in the commercial sector of Thailand. A computer tool for least-cost planning is used to formulate an alternative power-development plan. Features of the resultant plan include an 8% reduction in the required capacity addition compared to a reference plan.
The influence of collector aspect ratio on the 96lOl566 collector efficiency of baffled solar air heaters Yeh, H.-M. et al. Energy, 1998, 23, (1). 11-16. A theoretical investigation was carried out into the effect of collector aspect ratio on the collector efficiency of baffled solar air heaters. With constant collector area, the collector efficiency increases when the collector aspect ratio increases. This is the same results as those obtained in the previous work for flat-plate solar air heaters without fins and baffles. Although the collector efficiency of baffled solar air heaters is larger than that of flatplate heaters without fins and baffles, the improvement of collector efficiency by increasing the collector aspect ratio is reverse.
Space heating and cooling
Maintenance of efficient operation of primary gas
Yankovsknya, E. V. et al. Koks Khim., 1995, (9). 23-26. (In Russian) The paper discusses the significance and conditions of primary cooling of coking gas and the corrosion and salt deposition prevention.
Modelling of heat storage by solid/liquid phase 96iO1566 change with the first and second laws of thermodynamics Choquette, F. and Lacroix, M. Rev. Gen. ‘fherm., 1997, 36, (lo), 732-743. (In French) Energy storage was analysed by melting of a phase change material,(PCM) when the energy source is a stream of hot fluid. A numerical model based on the first and second laws of thermodynamic was developed to describe the energy conservation within the hot fluid and the PCM and the exergy destruction in the overall system. An increase in the mass flow rate of the hot fluid was revealed to accelerate the melting of the PCM leads to important exergy destruction within the overall system. Moreover, the optimal melting temperature of the PCM deviates from the expected geometric mean of the heat source and environment temperature. The problem of initial subcooling of the PCM was considered. The study also revealed that the effects of this initial condition vanish and become negligible when -40% of the PCM slab has melted.
9wo1571 Numerical simulation of a multi-layer latent heat thermal energy storage system
Brousseau, P. and Lacroix, M. Int. J. Energy Res., 1998, 22, (1) l-15. The paper presents a computational model for the prediction of the thermal behaviour of a compact multi-layer latent heat storage unit. The model is based on the conservation equations of energy for the phase change material (PCM) and the heat transfer fluid (HTF). Electrical heat sources embedded inside the PCM are used for heat storage (melting) while the flow of an HTF is employed for heat recovery (solidification). Parametric studies are performed to assess the effect of various design parameters and operating conditions on the thermal behaviour of the unit. Results indicate that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, on the thermal diffusivity of the liquid phase, on the thickness of the PCM layer and on the HTF inlet mass flow-rate and temperature. It is, on the other hand, nearly independent of the wall thermal diffusivity and thickness and of the maximum operating temperature. Correlations are proposed for the total energy stored and the output heat load as a function of the design parameters and the operating conditions.
9WO1572 Performance analysis of an irreversible Brayton heat engine
Chen, L. et al. .I. Institute of Energy, March 1997, 70, 2-8. The paper analyses the performance of an irreversible Brayton heat engine. Analytical formulae related to powe,r output, pressure ratio and efficiency of the heat engine coupled to either constant- or variable-temperature heat reservoirs are derived. The irreversibilities considered include heatresistance losses in the hot- and cold-side heat-exchangers, and nonisentropic expansion and compression losses in the turbine and compressor. The optimal performance characteristics of the heat engine are obtained by optimization of the distribution of heat conductances or heat-transfer surface areas between the heat-exchangers, and the matching of working fluid and heat reservoirs.
9wo1573 Primitive parts: an approach to air-conditioning component modelling Chow, T. T. et al. Energy and Buildings, 1997, 26, (2), 165-173. The conceptual design of the HVAC plant configurations vary dramatically, as do their complex building and plant topologies. Plant performance analysis often relies on a powerful and flexible simulation tool to produce realistic results based on appropriate system and component models. The credibility and applicability of contemporary systems simulation packages and the component libraries that underpin these systems are thus stressed. An approach to plant component modelling, based on the notion of ‘primitive parts’ (PP) to represent the fundamental heat and mass transfer processes is described. The principal advantage of the approach lies in its flexibility in relation to the multiplicity of plant systems, possible modelling abstractions and converging models at the theoretical level. The PP approach is described in terms of its features, applications, limitations and planned developments.
9wo1574 Re ression analysis of high-rise fully air-conditioned offlce bu$ ldings
Lam, J. C. et al. Energy and Building& 26, (2), 189-197. A parametric study of a generic high-rise air-conditioned office building in Hong Kong was performed with the building energy simulation computer program DOE-2. Twenty-eight design parameters related to the building load, the heating, ventilating and air-conditioning (RVAC) system and the HVAC refrigeration plant were found to correlate well with the predicted annual electricity consumption. Regression models and energy equations for the prediction of annual electricity use were developed via both linear and non-linear multiple regression techniques. Twelve input design parameters were considered to be the most significant design variables and were used in the energy prediction equations. The resulting models and energy equations can be used as a simplified design tool for comparing the
Fuel and Energy Abstracts