W.M. Worek

NUMERICAL SIMULATION OF COMBINED HEAT AND MASS TRANSFER PROCESSES IN A ROTARY DEHUMIDIFIER

A new finite-difference method is presented for simulating the combined heat and mass transfer processes that occur in a solid desiccant dehumidifier, which is the main component of a solid desiccant dehumidification and cooling system. The numerical method is of high-order accuracy, is numerically implicit, and is unconditionally stable. This method allows for more rapid simulation of the performance of desitcant-based dehumidification and cooling systems. Using the numerical method, the effect of the rotational speed on the performance of an adiabatic rotary dehumidifier was parametricaily studied, and the optimal rotational speed was determined by examining the outlet adsorption-side humidity profiles and the humidity wave fronts inside the desiccant dehumidifier.

The Performance of the Kalina Cycle System 11(KCS-11) With Low-Temperature Heat Sources

The possibility of exploiting low-temperature heat sources has been of great significance with ever increasing energy demand. Optimum and cost-effective design of the power cycles provide a means of utilization of low-temperature heat sources which might otherwise be discarded. In this analysis, the performance of the Kalina cycle system 11 (KCS11) is examined for low-temperature geothermal heat sources and is compared with an organic Rankine cycle. The effect of the ammonia fraction and turbine inlet pressure on the cycle performance is investigated in detail. Results show that for a given turbine inlet pressure, an optimum ammonia fraction can be found that yields the maximum cycle efficiency. Further, the maximum cycle efficiency does not necessarily yield the optimum operating conditions for the system. In addition, it is important to consider the utilization of the various circulating media (i.e., working fluid, cooling water, and heat resource) and heat exchanger area per unit power produced. For given conditions, an optimum range of operating pressure and ammonia fraction can be identified that result in optimum cycle performance. In general, the KCS11 has better overall performance at moderate pressures than that of the organic Rankine cycle.

Entropy Generation in Convective Heat Transfer and Isothermal Convective Mass Transfer

The irreversible generation of entropy for two limiting cases of combined forced-convection heat and mass transfer in a two-dimensional channel are investigated. First, convective heat transfer in a channel with either constant heat flux or constant surface temperature boundary conditions are considered for laminar and turbulent flow. The entropy generation is minimized to yield expressions for optimum plate spacing and optimum Reynolds numbers for both boundary conditions and flow rigimes. Second, isothermal convective mass transfer in a channel is considered, assuming the diffusing substance to be an ideal gas with Lewis number equal to unity. The flow is considered to be either laminar or turbulent with boundary conditions at the channel walls of either constant concentration or constant mass flux. The analogy between heat and mass transfer is used to determine the entropy generation and the relations for optimum plate spacing and Reynolds number. The applicable range of the results for both limiting cases are then investigated by non-dimensionalizing the entropy generation equation.

Optimization of wet-surface heat exchangers

Wet-surface heat exchangers are analyzed in order to determine the configuration that optimizes the performance. The objective is to cool a stream of air to a temperature lower than the inlet wet-bulb temperature by the evaporation of water. Three laboratory models and a commercial prototype were analyzed. They are a unidirectional, a counter-flow, a counter-flow closed-loop configuration and a cross-flow closed-loop commercial unit, respectively. It was found that dry-bulb temperatures considerably lower than the inlet wet-bulb temperature can be easily achieved. In fact, the inlet dew-point temperature can be approached with moderate flow rates and simple geometries.

Entropy generation in combined heat and mass transfer

Abstract Irreversible entropy generation for combined forced convection heat and mass transfer in a twodimensional channel is investigated. The heat and mass transfer rates are assumed to be constant on both channel walls. For the case of laminar flow, the entropy generation is obtained as a function of velocity, temperature, concentration gradients and the physical properties of the fluid. The analogy between heat and mass transfer is used to obtain the concentration profile for the diffusing species. The optimum plate spacing is determined, considering that either the mass flow rate or the channel length are fixed. For the turbulent flow regime, a control volume approach that uses heat and mass transfer correlations is developed to obtain the entropy generation and optimum plate spacing.

Parametric study of an open-cycle adiabatic, solid, desiccant cooling system

The performance of an open-cycle, adiabatic, solid, desiccant cooling system operating in the ventilation mode is modeled numerically. The effects of nondimensional dehumidifier channel length, desiccant mass fraction and desiccant isotherm shape are investigated. The results yield conditions for which the system has an optimum thermal coefficient of performance (COP).

Simulation of a regenerative, closed-cycle adsorption cooling/heating system

A regenerative, closed-cycle adsorption cooling/heating system with performance significantly higher than conventional adsorption systems is modeled numerically. The equations governing the transport of heat and mass that occurs in such systems are derived, and the system performance is determined by using both infinite and finite transport coefficients. The effects of varying the evaporator and condenser temperatures and the amount of inert material in the system on overall system performance also are presented.

INFLUENCE OF ELEVATED PRESSURE ON SORPTION IN DESICCANT WHEELS

Rotary desiccant wheels have been employed to dry pressurized air streams. In these systems, depending on the moisture content in the air stream and the operating pressure, condensation can occur in the regeneration portion of the wheel. In this article, a numerical method using an implicit finite-difference scheme is developed and applied that enables condensation to be detected and simulated in the regeneration portion of a desiccant wheel operating at high pressures. Using this model, performance analysis of desiccant wheel under these conditions is investigated. It is found that, depending on the value of the separation factor and regeneration temperature, condensation could occupy as much as 40% of regeneration section of the wheel. In this region, regeneration of the desiccant is not possible and usually dehumidification of regeneration air occurs. Also, as the operating pressure increases, the adsorption and desorption characteristics are dramatically affected and the optimum separation factor of desiccant material increases with operating pressure.

Effect of Design and Operating Parameters on the Performance of Two-Bed Sorption Heat Pump Systems

The effect of design and operating parameters on the performance of closed-cycle, two-bed sorption heat pump systems were investigated. The parameters studied in this paper included the effects of bed switching frequency (i.e., the switching speed), the sorbent bed NTU (Number of Transfer Units), the thermal resistance within the sorbent, the contact resistance between the sorbent and the tube wall of the heat transfer fluid, the fraction of inert mass within the sorbent bed heat exchanger, and the amount of fluid resident within the heat exchanger. The results show that the performance of a sorption heat pump system is extremely sensitive to the switching speed. In fact, the value of the switching speed that optimizes the COP is different from the value that optimizes the overall cooling capacity. Such a performance characteristic allows the design of multispeed sorbent bed heat pumps to be operated in such a way to follow the load while still maximizing the system COP. However, operation of the system at switching speeds away from the optimum values can cause a dramatic deterioration in system performance.

Thermal contact resistance across a copper-silicon interface.

An experimental setup to measure the thermal contact conductance across a silicon-copper (Si-Cu) interface is described, and the results obtained are presented. The resulting thermal contact resistance data are used in estimating the thermo-mechanical and optical performance of optical substrates cooled by interfaced copper cooling blocks. Several factors influence the heat transfer across solid interfaces. These include the material properties, interface pressure, flatness and roughness of the contacting surfaces, temperature, and interstitial material, if any. Results presented show the variation of thermal contact conductance as a function of applied interface pressure for a Cu-Si interface. Various interstitial materials investigated include indium foil, silver foil and a liquid eutectic (Ga-In-Sn). As expected, thermal contact resistance decreases as interface pressure increases, except in the case of the eutectic, in which it was nearly constant. The softer the interstitial material, the lower the thermal contact resistance. Liquid metal provides the lowest thermal contact resistance across the Cu-Si interface, followed by the indium foil, and then the silver foil.