Sushil K. Chaturvedi

Thermal performance of a variable capacity direct expansion solar-assisted heat pump

A variable capacity direct expansion solar-assisted heat pump system is developed and operated for domestic hot water application. The proposed system employs a bare solar collector which also acts as the system evaporator. A variable frequency drive modulates the compressor speed to maintain a proper matching between the heat pumping capacity of the compressor and the evaporative capacity of the collector under widely varying ambient conditions. Experimental results indicate that the coefficient of performance of the system can be improved significantly by lowering the compressor speed as ambient temperature rises from winter to summer months. Experimentally measured system COPH and water side heat capacity rate are also in good agreement with theoretical predictions.

Thermal performance of a direct expansion solar-assisted heat pump

Abstract A direct expansion solar assisted heat pump, in which a bare flat plate collector also acts as the evaporator for the refrigerant, Freon-12, is designed and operated. The system components, e.g. the collector and the compressor, are properly matched so as to result in system operating conditions wherein the collector/evaporator temperature ranges from 0 to 10°C above ambient temperature under favorable solar conditions. This operating temperature range is particularly favorable to improved heat pump and solar collector performance. The system thermal performance is determined by measuring refrigerant flow rate, temperature and pressure at various points in the system. The heat pump COP H and the solar collector efficiency ranged from 2.0 to 3.0 and from 40 to 70 per cent, respectively, for widely ranging ambient and operating conditions. Experimental results indicate that the proposed system offers significant advantage in terms of superior thermal performance when compared with results gotten by replacing the solar evaporator with a standard outdoor fan-coil unit.

Thermodynamic analysis of two-component, two-phase flow in solar collectors with application to a direct-expansion solar-assisted heat pump

Two-phase flow of pure chlorofluorocarbon (CFC) refrigerants in solar collector tubes has been examined in previous studies in connection with applications in direct-expansion, solar-assisted heat pumps (DX-SAHP). The present work extends the thermodynamic analysis of solar collectors to the multicomponent and multiphase domain to cover newly proposed refrigerant mixtures which are potential candidates for replacing CFCs in future DX-SAHP systems. A computational methodology is developed to determine the size of a solar collector of a DX-SAHP that uses a binary refrigerant mixture whose thermodynamic and transport properties are predicted from a computer code. The energy equation for the elemental collector tube control volume, incorporating the local thermodynamic and heat transfer characteristics, is integrated to determine the tube length for a given set of inlet and exit thermodynamic states of the refrigerant mixture. Effects of various parameters such as the collector mass-flow rate and operating pressure, tube diameter and absorbed solar radiation on the collector tube length, heat transfer coefficient, and the local refrigerant temperature in the tube are also considered.

Analysis of Two-Phase Flow Solar Collectors With Application to Heat Pumps

A thermodynamic model is developed to analyze the thermal performance of twophase solar collectors. The well-known equilibrium homogeneous theory is used to model the two-phase flow in the solar collectors. The resultant set of coupled ordinary differential equations for saturated pressure and quality of working fluid in the collector tubes are solved by an iterative procedure using a fourth-order RungeKutta method. The results are then applied to determine the thermal performance of a solar assisted heat pump which uses two-phase flow collectors as the evaporator. The results indicate that even with the use of less expensive bare solar collectors as evaporator for the heat pump, the heating coefficient of performance (COP /SUB H/) as high as 6 can be obtained under realistic ambient conditions provided a proper matching exists between the collectors evaporative capacity and the compressors pumping capacity.

Transient simulation of a capacity-modulated, direct-expansion, solar-assisted heat pump

Abstract The long-term thermal performance of a direct-expansion, solar-assisted heat pump is determined from the transient simulation of the system. The system employs a bare collector that also acts as the heat pump evaporator. Of particular interest in this study is the configuration in which the compressor and the collector area are properly matched from the long-term thermal performance point of view. This matching is achieved through multistep as well as two-step compressor capacity modulation. In addition to examining the effects of compressor capacity modulation, the effects of various system parameters such as collector area, storage volume, load temperature, wind speed, collector slope, and refrigerant properties are also studied in detail. Monthly averaged thermal performance parameters such as the heat pump system coefficient of performance are determined by executing a computer simulation program that uses the typical meterological year (TMY) solar data for Norfolk, Virginia. Results indicate that the system performance is governed strongly by collector area, compressor RPM, load temperature, and refrigerant properties. The remaining parameters have only weak influence on the long-term system performance of direct expansion solar-assisted heat pump (SAHP) system considered in this study.

An approximate method for calculating laminar natural convective motion in a trombe-wall channel

Laminar natural convective motion in a channel formed by differentially heated vertical plates is analyzed. The proposed model combines the momentum-integral equation with the Oseen approximation for convective terms in the energy equation to predict the volumetric flow rate as a function of channel height. A second-order ordinary differential equation for pressure defect in the channel is derived by approximating the axial velocity profile with a fourth-order polynomial. Results obtained from the present model are in good agreement with previously reported results. Consideration of a second-order axial velocity profile in the momentum-integral model leads to closed form solutions that are in good agreement with previously reported results only in the mid to high flowrate regime. In the low flow-rate regime, the second-order model gives results that deviate significantly from results obtained for other models. Neglect of inertia terms in the momentum-integral model leads to a first-order differential equation for the pressure and a closed form solution of the problem. However, this approximation yields results that are good only in the mid to high flowrate regime, while showing deviations from other models in the low flowrate regime. Finally, the present model is also shown to be capable of application to fluids with widely ranging Prandtl numbers.

Numerical prediction of pressure loss coefficient and induced mass flux for laminal natural convective flow in a vertical channel

The natural convection heat transfer and ventilation characteristics of heated and vented parallel wall channels have been studied numerically. The flow is assumed to be laminar and steady, and the governing two-dimensional Navier-Stokes equations are solved by a finite volume formulation to calculate the chimney effect that draws cooler ambient air from the lower opening. The fluid-dynamic and heat-transfer characteristics of vented vertical channels are investigated for both symmetric isothermal and constant heat-flux boundary conditions for the Rayleigh number ranging from 103 to 105 and the aspect ratio in the 5–20 range. The non-dimensional entrance and exit pressure losses and the induced mass-flow are correlated with the Rayleigh number. The results indicate that although inlet- and exit-loss coefficients may vary significantly with Rayleigh number and aspect ratio, the total pressure-loss coefficient is a weak function of Rayleigh number. It is also shown that the total pressure loss coefficient and non-dimensional mass-flow rate results are better correlated with the modified Rayleigh number that is obtained by dividing the Rayleigh number by the aspect ratio. The elliptic flow results, obtained from the present procedure, are compared with fully developed flow results and with boundary-layer calculations of previous authors. Numerical results also yielded important information regarding the placement of the free pressure boundary. The results for the present geometry indicate that the effect of free-boundary location is negligible if it is placed at a distance of four times the channel width or greater.

Second-law analysis of solar-assisted heat pumps

In the present study, the first and second laws of thermodynamics are applied to solar-assisted heat-pump systems. Several relations between first- and second-law efficiency parameters (such as the coefficient of performance, primary energy ratio, exergetic and collector efficiencies) are derived. These relations are used to determine the system performance under realistic operating conditions. The performance of a solar-assisted heat-pump system is compared with that of a solar system without heat-pump. The sources of irreversibilities in the system are identified and their effects on system performance are discussed.

Heat and mass transfer from a supercritical lox spray

Abstract The injection, evaporation and diffusion of liquid oxygen in a high pressure airstream in a parallel wall mixing channel is analyzed and computationally solved. The droplet evaporation in the supercritical environment is treated by a non-isothermal droplet heat transfer model which accounts for the finite thermal conductivity of oxygen droplets and the gas film. The non-ideal gas effects in the gas phase are modeled by the Redlich-Kwong equation of state. The mixture density and enthalpy are determined by applying the ideal-solution limit which is shown to be valid for the prevailing conditions. The coupled dynamics of droplet and gas phases is calculated by solving numerically the Navier-Stokes equations in two dimensions. The turbulence effects are modeled by a two equation (k-ϵ) model. The results show that the non-ideal gas behavior prevails over a large portion of the mixing channel. Furthermore, the injected liquid oxygen droplets achieve critical temperature very quickly, and as a result they evaporate in the vicinity of the injection point. The effects of injection angle on oxygen mixing characteristics is also investigated.

Unit Thermal Performance of Atmospheric Spray Cooling Systems

Thermal performance of an open atmospheric spray pond or canal depends on the direct-contact evaporative cooling of an individual spray unit (spray nozzle or module) and the interference caused by local heating and humidification. Droplet parameters may be combined into a dimensionless group, number of transfer units (NTU) or equivalent, whereas large-scale air-vapor dynamics determine interference through the local wet-bulb temperature. Quantity NTU were implied from field experiments for a floating module used in steam-condenser spray canals. Previous data were available for a fixed-pipe nozzle assembly used in spray ponds. Quantity NTU were also predicted using the Ranz-Marshall correlations with the Sauter-mean diameter used as the characteristic length. Good agreement with experiments was shown for diameters of 1–1.1 cm (module) and 1.9 mm (fixed-pipe nozzle).