Eric B. Ratts

Entropy generation minimization of fully developed internal flow with constant heat flux

We use the entropy generation minimization (EGM) method to optimize a single-phase, convective, fully developed flow with uniform and constant heat flux. For fixed mass flow rate and fixed total heat transfer rate, and the assumption of uniform and constant heat flux, an optimal Reynolds number for laminar and turbulent flow is obtained. The study also compares optimal Reynolds number and minimum entropy generation for cross sections: square, equilateral triangle and rectangle with aspect ratios of two and eight. The rectangle with aspect ratio of eight had the smallest optimal Reynolds number the smallest entropy generation number, and the smallest flow length

An experimental analysis of cycling in an automotive air conditioning system

Abstract In the majority of automotive air conditioning systems, the compressor continuously cycles on and off to meet the steady-state cooling requirements of the passenger compartment. Since the compressor is a belt-driven accessory device coupled to the engine, its cycling rate is directly related to the vehicle speed. The refrigeration system’s losses increase with increasing vehicle speed and thus with increasing compressor cycling. This paper identifies and quantifies individual losses in an automotive vapor-compression refrigeration system during compressor cycling. The second law of thermodynamics, in particular, nondimensional entropy generation, is used to quantify the thermodynamic losses of the refrigeration system’s individual components under steady driving conditions at idle, 48.3 kph (30 mph), and 96.6 kph (60 mph). A passenger vehicle containing a cycling-clutch orifice-tube vapor–compression refrigeration system was instrumented to measure refrigerant temperature and pressure, and air temperature and relative humidity. Data were collected under steady driving conditions at idle, 48.3 kph (30 mph), and 96.6 kph (60 mph). A thermodynamic analysis is presented to determine the refrigeration system’s performance. This analysis shows that the performance of the system degrades with increasing vehicle speed. Thermodynamic losses increase 18% as the vehicle speed changes from idle to 48.3 kph (30 mph) and increase 5% as the vehicle speed changes from 48.3 kph (30 mph) to 96.6 kph (60 mph). The compressor cycling rate increases with increasing vehicle speed, thus increasing the refrigeration system’s losses. The component with the greatest increase in thermodynamic losses as a result of compressor cycling is the compressor itself. Compressor cycling reduces the compressor’s isentropic efficiency, and thus the system’s thermodynamic performance. The individual component losses of the refrigeration system are quantified. The redistribution of these losses is also given as a function of increasing vehicle speed (i.e. increasing compressor cycling). At 96.6 kph (60 mph), the thermodynamic losses, based on the ratio of entropy generation to entropic load, are 0.22, 0.10, 0.07, and 0.02 in the compressor, the condenser, the evaporator-accumulator, and the orifice tube, respectively. The compressor losses dominated the overall system performance. The overall system efficiency could be significantly improved by increasing the compressor’s efficiency. The compressor’s efficiency could be improved by reducing or eliminating cycling, such as could be accomplished by using a variable capacity compressor or by not directly coupling the compressor to the engine. Another way to increase the compressor’s volumetric efficiency during cycling would be to reduce the compressor operating range. This could be accomplished by using two compressors such as is done in two-stage cascade refrigeration systems.

A generalized analysis for cascading single fluid vapor compression refrigeration cycles using an entropy generation minimization method

Abstract This paper focuses on cascading an ideal vapor compression cycle and determining the optimal intermediate temperatures based on the entropy generation minimization method. Only superheating and throttle losses of the cycle are considered since they are inherent to the ideal vapor compression refrigeration cycle. The second law equations have been developed in terms of specific heats and temperature ratios with the intent of reducing involved property modeling. Also the entropy generation was expressed in terms of a single independent variable and minimized to develop an advanced rule for selecting optimum intermediate temperatures. Results for a cascade system operating between reduced temperatures of 0.684 and 0.981 with R-134a as the working fluid are presented. The approximate method presented here predicted the optimum intermediate reduced temperature for a two-stage system to be 0.88, a difference of 2% from the optimum. The method presented was a much better predictor of the optimum temperature than the geometric mean method which was 0.82, a difference of 5% from the optimum. The entropy generation distribution of the optimum solution was investigated. For a two-stage system, the lower stage and higher stage entropy generation was 44% and 56%, respectively. In comparison to the single stage, the two-stage reduced losses by 78%.

Thermal modeling of controlled atmosphere brazing process using virtual reality technology

Abstract In the automotive industry, heat exchangers are manufactured in large quantities. The controlled atmosphere brazing (CAB) process is one of the fastest growing processes for aluminum radiator production behind vacuum brazing and machine assembly process. In this paper, we describe a thermal model (HETCAB) to predict the transient temperature distribution in an aluminum heat exchanger while it is being brazed in a CAB furnace. This thermal model is simulated using a virtual CAB (VR CAB) furnace created using virtual reality technology. Within this VR CAB furnace, engineers can “walk” through the furnace, observe the dynamic heat exchange, manipulate the products inside the furnace, and test various parameters critical to the process. The VR CAB together with the thermal model provides a realistically simulated environment that enables engineers to control and improve the heat exchange processes, experiment with design parameters, and study the effects of various process parameters including the parameters that control product yield.

A Study of the Radiant Heat Exchange within a Controlled-Atmosphere Furnace

Transient three-dimensional heat transfer between a traversing, structured, and rectangular object and an enclosure is studied. This study investigates the heat transfer process that occurs in brazing an aluminum heat exchanger in a controlled-atmosphere furnace. A models development is discussed with prescribed enclosure temperature boundary conditions. The program determines the radiant heat exchange between gray diffuse surfaces, and solves the three-dimensional conduction equation for a solid with a radiant heat flux boundary condition using an implicit finite-difference method. The structured objects conduction and radiant thermal properties are described by effective values. It was shown that radiative thermal properties of the traversing object and the enclosures temperature have a strong impact on the objects temperature history. The effective thermal emissivity was found to influence the objects rate of temperature change. The enclosures temperatures influenced the objects equilibrium temperature. Also, it was shown that the objects position and rotation can alter its temperature distribution, but not as strong as the effect of boundary conditions and thermal properties. In addition to numerical methods, experiments were performed to further understand the process.

Thermal performance of ventilated passenger seats

Automotive seats are now actively cooled as well as heated to provide thermal comfort. Some seats are cooled by thermoelectric devices and others through simple forced ventilation. To ultimately determine passenger comfort requires knowing the seats ability to remove water vapor and thermal energy from the passenger’s skin. This paper presents work on measuring the mass and heat transfer performance of a forced-air ventilated seat. Comparing the thermal behavior to a semi-infinite body, an effective thermal effusivity was measured as well as an effective mass diffusivity. Average thermal effusivities were 64 and 244 W s1/2 /m2 K for the non-ventilated and ventilated seats, respectively. Average mass diffusivities were 0.0031 and 0.0142 m2 /s for the non-ventilated and ventilated seats, respectively.Copyright

Laminar entropy generation over a flat plate with isothermal and constant heat flux boundary conditions using the von Kármán integral method

This paper is a fundamental study on the irreversibility of single-phase laminar convective heat transfer over a flat plate with isothermal and constant heat flux boundary conditions. It quantifies the losses due to viscous momentum transfer losses and heat transfer losses and presents the irreversibility of the convective flow based on the entropy generation (EG) method. This paper determines the entropy generation for incompressible, single phase, laminar flow for large and small Prandtl numbers over a flat plate with isothermal and constant heat flux boundary conditions using von Karman’s integral theory.Copyright

Entropy generation minimization of fully developed internal convective flows with constant heat flux

This paper addresses the thermodynamic optimum of single-phase convective heat transfer in fully developed flow for uniform and constant heat flux. The optimal Reynolds number is obtained using the entropy generation minimization (EGM) method. Entropy generation due to viscous dissipation and heat transfer dissipation in the flow passage are summed, and then minimized with respect to Reynolds number based on hydraulic diameter. For fixed mass flow rate and fixed total heat transfer rate, and the assumption of uniform heat flux, an optimal Reynolds number for laminar as well as turbulent flow is obtained. In addition, the method quantifies the flow irreversibilities. It was shown that the ratio of heat transfer dissipation to viscous dissipation at minimum entropy generation was 5:1 for laminar flow and 29:9 for turbulent flow. For laminar flow, the study compared non-circular cross-sections to the circular cross-section. The optimal Reynolds number was determined for the following cross-sections: square, equilateral triangle, and rectangle with aspect ratios of two and eight. It was shown that the rectangle with the higher aspect ratio had the smallest optimal Reynolds number, the smallest entropy generation number, and the smallest flow length.Copyright