J. V. C. Vargas

Optimal allocation of a heat-exchanger inventory in heat driven refrigerators

This paper reports the thermodynamic optimization (or entropy generation minimization) of a heat-driven refrigeration plant, that is, a refrigerator without work input, which is driven by a heat source. The treatment accounts for the heat transfer irreversibilities of the three heat exchangers, and for the finiteness of the total heat-exchanger inventory. The operating conditions for maximum refrigeration rate are determined. It is shown that the heat-exchanger inventory must be divided optimally between the three heat exchangers. For example, half of the inventory must be placed in the heat exchanger used to reject heat to the ambient. The maximum refrigeration rate per unit of total heat exchanger inventory is reported. These thermodynamic optimization principles are then applied to a refrigerator driven by heat transfer from a solar collector.

Simulation in transient regime of a heat pump with closed-loop and on-off control

Abstract The present work introduces a mathematical model for a heat pump with a variable-speed compressor, driven by a d.c. servomotor, operating either in closed loop by a power law control action or by the traditional on-off basis. The resulting differential and algebraic equations are integrated in time for a specified period of simulation in both designs. The results show that the closed-loop system presents significant savings in energy consumption when compared with the on-off system, under the same environmental conditions.

Thermodynamic optimization of finned crossflow heat exchangers for aircraft environmental control systems

Abstract This paper shows that the main geometric features of a flow component can be deduced from the thermodynamic optimization of the global performance of the largest flow system that incorporates the component. This approach represents a departure from the usual approach, where a flow component is optimized in isolation. The example chosen is the counterflow heat exchanger of the environmental control system (ECS) used on modern aircraft. The heat exchanger is fitted with a diffuser and a nozzle for the ram air, and the ECS runs on the boot strap air cycle, employing an additional compressor and turbine. Two heat transfer surface types are considered, finned and smooth parallel plates. Numerical results are reported for the external geometric aspect ratios of the heat exchanger, and for the plate-to-plate spacing of the smooth-plates model. It is shown that the optimized geometry for the core with finned surfaces is nearly the same as the optimized geometry for the core with smooth plates. Several of the optimized geometric features are robust with respect to changes in external parameters that vary from one application to the next. The method illustrated in this paper – the thermodynamic (constructal) optimization of flow geometry – is applicable to any system that runs on the basis of a limited amount of fuel (exergy) installed onboard, e.g., automobiles, ships, portable tools.

Two design aspects of defrosting refrigerators

Abstract This paper shows that the intermittent operation of a defrosting vapour-compression-cycle refrigerator can be optimized with respect to: (1) the frequency of on/off operation, and (2) the way in which the supply of heat exchanger surface is divided between evaporator and condenser. The method used is that of thermodynamic optimization (or entropy generation minimization, or finite-time thermodynamics), in which heat transfer and thermodynamic aspects are accounted for simultaneously to produce a realistic description of the in-time operation of the installation. The optimal on/off frequency and surface allocation ratio are reported in non-dimensional charts, which show the effect of the heat exchanger size, cycle temperature ratio, defrosting time, compressor efficiency, and refrigerant type. The charts are based on the real properties of refrigerants R12 and R134a. The heat exchanger equipment constraint is that of fixed total heat transfer surface. It is shown that, under certain conditions, the optimization conclusions reached in this study are similar to those that would be obtained based on other constraints proposed in the literature.

When to defrost a refrigerator, and when to remove the scale from the heat exchanger of a power plant

Abstract This paper demonstrates the existence of an optimal on/off sequence for operating a household refrigerator that accumulates ice on its evaporator coils. Experimentally it is shown that the rate of ice formation is constant in time. The optimal (intermittent) regime of operation determined in this paper minimizes the power required by the refrigerator, maintains the prescribed temperature of the cold space, and removes intermittently the ice layer. The second part of the paper proves that a similar strategy can be used for dealing with fouling in heat exchangers, i.e. for maximizing the power output of a power plant that is diminished by the formation of scale on its heat exchanger surfaces. These optimal on/off sequences of operation for refrigerators and power plants are proven based on pure heat transfer and thermodynamics grounds. The refrigerator and power plant models employed are the simplest models possible. Consequently, the optimal on/off sequences demonstrated in this paper are fundamental features that will be present (and deserve to be identified and exploited) in the design of actual refrigerators and power plants, no matter how complicated these designs may be.

Power extraction from a hot stream in the presence of phase change

This paper considers the basic thermodynamic optimization problem of extracting the most power from a stream of hot exhaust when the contact heat transfer area is fixed. It shows that when the receiving (cold) stream boils in the counterflow heat exchanger, the thermodynamic optimization consists of locating the optimal capacity rate of the cold stream. At the optimum, the cold side of the heat transfer surface divides itself into three sections: liquid preheating, boiling and vapor superheating. Numerical results are developed for a range of design parameters of applications with either water or toluene on the cold side. It is shown that the optimal design is robust, because several of the design parameters have only a weak effect on the optimal design.

Integrative thermodynamic optimization of the environmental control system of an aircraft

Abstract In this paper we propose to optimize the geometric configuration of a component by maximizing the global thermodynamic performance of the much larger system that contains the component. This “integrative” approach departs from current thermodynamic optimization practice in which the configuration of a component (e.g., heat exchanger) is optimized by itself, in isolation. In the present example the larger system is an aircraft and the component is its environmental control system (ECS). We show that the configuration of the ECS impacts the performance (exergy destruction, fuel consumption) of the aircraft in two ways, not one: through its own irreversibility, and its weight-related contribution to the power required to sustain the flight. By minimizing the thermodynamic losses at the aircraft level, we deduce all the geometric details of the cross-flow heat exchanger that dominates the weight and structure of the ECS. The optimized geometry is robust with respect to changes in some of the operating parameters that have to be specified. The integrative method illustrated in this paper is generally applicable to the optimization of architecture in other systems where all the functions are driven by the exergy of the fuel installed onboard.

The Melting of an Ice Shell on a Heated Horizontal Cylinder

This paper describes the fundamentals of melting when a shell of phase-change material rides on a heated horizontal cylinder. In the first part of the paper, contact melting theory is used to predict the history of the melting process and, in particular, the time when the remaining ice falls off the cylinder. It is shown that the melting process consists of two distinct regimes, first, an early regime when the cylinder is surrounded by ice and, second, a late regime when the cylinder cuts through the top of the ice shell. The second part describes laboratory measurements that validate the theory. The third part of the paper shows that in the complete cycle that starts with freezing the shell and ends with the contact-melting removal of the shell, there exists an optimal frozen shell thickness such that the cycle-averaged production of ice is maximized.

Nonsimilar solutions for mixed convection on a wedge embedded in a porous medium

The problem of mixed convection on a wedge in a saturated porous medium is analyzed using the Darcy flow formulation and three different methods of solution. Nonsimilar solutions are obtained for several wedge angles. The nonsimilarity technique is applied to the boundary layer formulation, and the finite element method is used in both formulations. It is shown that both formulations produce results that agree well for Pe = 1 and uniform wall temperature in the range 0.1 m = 1 /3, 1/2 and 1 (i.e., wedge half angles 7 = 45, 60, and 90). It is shown that the overall heat-transfer rate is the largest when the wedge angle is zero, and the walls are oriented vertically.

Notional all-electric ship thermal simulation and visualization

The impact of unpredicted or unplanned thermal disturbances on any future all-electric ship may well lead to unexpected and untimely failure of mechanical-electrical systems (e.g., power electronics, high power sensors, and pulsed weapons) to the detriment of the ships combat mission. The high power, rapid transients, and harsh environment expected to be imposed on both the electrical and thermal systems may well be unique to this class of ship. In order to develop the thermal analysis, a comprehensive visualization tool to display the temperature and heat dissipation distributions in the entire ship has been developed. This tool includes a simplified physical model, which combines principles of classical thermodynamics and heat transfer, resulting in a system of three-dimensional differential equations which are discretized in space using a three-dimensional cell centered finite volume scheme. Therefore, the combination of the proposed simplified physical model with the adopted finite volume scheme for the numerical discretization of the differential equations is called a volume element model (VEM). In this work, a 3D simulation is performed in order to determine the temperature distribution inside the ship for six different operating conditions. Visit visualization tool is used to plot the results.