Analytical study of the endface friction of the revolving vane mechanism

The effects of the fluid friction acting on the endfaces of the revolving vane (RV) machine, where the cylinder rotates together with the rotor, are investigated analytically with closed form solutions. It was found that the endface losses of the RV machine are generally affected by both the relative velocity and the eccentricity between the rotor and the cylinder. The endface loss of an RV machine with the simply supported bearing arrangement is found to be always lower than that with the cantilever arrangement.When compared to a similar rotary machine but with a stationary cylinder, the RV machine used in this study exhibits a 95% decrease in the endface loss, while in the wider practical dimensions range, an RV machine consistently shows at least a 50% lower in endface loss.These findings supply more proof to the claim of the suitability of the employment of the RV mechanism in high-speed and high-pressure applications.     

Microchannel component technology for system-wide application in ammonia/water absorption heat pumps

A novel miniaturization technology for binary-fluid heat and mass exchange was developed and numerous components were fabricated for demonstration as different parts of an ammonia/water absorption heat pump. Short lengths of microchannel tubes are placed in an array, with several such arrays stacked vertically. The ammonia/water solution flows in falling film/droplet mode on the outside of the tubes while coupling fluid flows through the microchannels. Coupling fluid heat transfer coefficients are extremely high due to the use of microchannel tubes. Effective vapor-solution contact on the absorption side minimizes heat and mass transfer resistances. This concept addresses all the requirements for absorber design in an extremely compact geometry. The technology is suitable for almost all absorption heat pump components (absorbers, desorbers, condensers, rectifiers, and evaporators) and for a wide range of binary-fluid processes. The development of several components for absorption and desorption at different capacities using this technology is reported here.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.     

Analytical solution for the simultaneous heat and mass transfer problem in air washers

An analytical solution approach for the simultaneous heat and mass transfer problem in air washers operating as evaporative coolers is presented. A one-dimensional model using the coupled mass and energy balance equations in the air washer is presented. Then, starting from a linear approach for the experimental curve of the air saturation, an analytical solution for the model was derived. The solution showed an excellent agreement with the available results found in the literature. The influence of several important parameters for the cooling process such as temperature and ambient air humidity, air flow rate and feeding water temperature, in the air cooling rate was investigated. The efficacy of the process can be greatly increased by reducing the cooling water temperature and the applied air flow rate. The analytical solution can be easily included into the models used for simulating desiccant air-conditioning systems operating in conjunction with air washers.     

Mathematical analysis for the efficiency of a semi-spherical fin with simultaneous heat and mass transfer

In this paper an analytical solution for the efficiency of a semi-spherical fin when subjected to simultaneous heat and mass transfer mechanisms is studied. For the mathematical analysis of a wet fin equation, a relationship between humidity ratio and temperature of the saturation air is needed. The driving forces for the heat and mass transfer are the temperature and humidity ratio differences, respectively. Analytical solutions are obtained for the temperature distribution over the fin surface when the fin is fully wet. It is observed that in humid conditions the fin has high efficiency to be used in industry. The variation effects of these parameters have been considered. Finally linear relation has been proposed for humidity and temperature on the fin surface.     

Transcritical carbon dioxide microchannel heat pump water heaters: Part II - System simulation and optimization

This paper presents the development of a transcritical CO₂ heat pump water heating system model incorporating analytical heat exchanger models and an empirical compressor model. This study investigated the effects of a suction line heat exchanger (SLHX) and once-through versus recirculating water heating schemes. The once-through systems outperformed the recirculating systems by 10% for the system without an SLHX and 15% with an SLHX. However, a gas cooler twice as large is required. The SLHX was shown to benefit system performance at higher evaporator temperatures with improvements of 16.5% for the once-through and 4% for the recirculating systems. This study can be used to improve the design of microchannel based transcritical CO₂ heat pumps; evaluate the impact of varying water inlet temperature, desired outlet temperature and evaporation temperature on system performance; and quantify the effect of differential diurnal electricity rates on system operating costs for these different operation schemes.     

Experimental investigation of the surface temperature and water retention effects on the frosting performance of a compact microchannel heat exchanger for heat pump systems

Frost formation on a louvered fin microchannel heat exchanger was experimentally investigated in this paper with the aim of determining the dominant factors affecting the time of frosting and frost growth rate. A novel methodology was developed to measure frost thickness and frost weight at intervals during the frosting period. Frost mass and thickness growth rates, corresponding coil heat transfer, capacity degradations and air pressure drop are measured and discussed. The experimental data showed that at a given air dry bulb temperature, the fin surface temperature and air humidity are the primary parameters that influence the frost growth rates. Water retention and air velocity had a secondary impact on the frosting performance. From digital images of the frost growth it was observed that frost does not nucleate from the water droplets retained in between fins but it developed from the leading edges of the fins.     

Experimental techniques to determine the convective heat transfer coefficients for flat and curved surfaces

This paper surveys the techniques which are available or being developed for measuring local and mean convective heat transfer coefficients, primarily for forced convection between a surface ,and an incompressible gas. After defining the heat transfer coefficient, the paper summarizes methods of surface temperature measurement, outlines the direct and indirect methods of determining the heat transfer coefficient and compares the pertinent features of three techniques suitable for use during film and multijet impingement cooling and with annular ducts.     

Transcritical carbon dioxide microchannel heat pump water heaters: Part I - validated component simulation modules

An experimental and analytical study on the performance of carbon dioxide heat pumps for water heating was conducted. The performance of compact, microchannel, water-coupled gas coolers, evaporator, and suction line heat exchanger (SLHX) were evaluated in an experimental facility. Analytical heat exchanger models accounting for the flow orientation and changing CO₂ thermophysical properties were developed and validated with data. Heat transfer coefficients were predicted with correlations available in the literature and local heat duty calculated using the effectiveness-NTU approach. The gas cooler, evaporator, and SLHX models predicted measured heat duties with an absolute average error of 5.5%, 1.3%, and 3.9%, respectively. Compressor isentropic and volumetric efficiency values were found to range from 56% to 67% and 62%-82%, respectively. Empirical models for compressor efficiency and power were developed from the data. The resulting component models are implemented in a system model in a companion paper (Part II).     

The role of surfactant adsorption rate in heat and mass transfer enhancement in absorption heat pumps

The importance of heat and mass transfer additives in absorption chillers and heat pumps has been recognized for over three decades. However, a universally accepted model for the mechanisms responsible for enhanced absorption rates has yet to be proposed. The Marangoni effect—an instability arising from gradients in surface tension at the liquid-vapor interface—is generally accepted as the cause of the convective flows that enhance transfer rates. Certain surfactant additives can significantly improve absorption rates and thus reduce the overall transfer area required by a given machine. Any means available that can increase the efficiency and acceptability of absorption machines is to be welcomed, as this technology provides an alternative to vapor compression systems which is both environmentally friendly and more versatile with regards to energy sources. This study investigates the rate at which a surfactant additive adsorbs at a liquid-vapor interface. The residence time of the falling liquid solution in an absorber is quite short. An effective additive must not only reduce the surface tension of the solution; it must do so quickly enough to cause the Marangoni instability within the short absorption process time. The entrance region of an absorber features a freshly exposed interface at which no surfactant has adsorbed. A numerical model is used to analyze surfactant relaxation rates in a static film of additive-laced solution. Kinetic parameters for the combination of the working pair LiBr-H₂O and the additive 2-ethyl-1-hexanol are derived from data in the literature for static and dynamic surface tension measurements. Bulk, interfacial and boundary parameters influencing relaxation rates are discussed for surfactant adsorption occurring in the absence of absorption, as well as for concurrent adsorption and stable vapor absorption. Initial solution conditions and absorption driving force are shown to impact the potential for instability in the effect they have on the rate of interfacial additive adsorption.     

Water-coupled carbon dioxide microchannel gas cooler for heat pump water heaters: Part I - Experiments

An experimental and analytical study on the performance of a compact, microchannel water- carbon dioxide (CO₂) gas cooler was conducted. The gas cooler design under investigation used an array of serpentine refrigerant microchannel tubes wrapped around water passages containing offset strip fins, resulting in a generally counterflow configuration between the two fluids. Part I of this two-part paper addresses the experimental aspects. Data were obtained using an experimental heat pump facility at varying inlet conditions for three gas coolers of the same design, but different sizes. Measured heating capacity for the three gas coolers ranged from 1.5 to 6.5 kW. The results of this study are used in the companion paper (Part II) to develop a predictive heat exchanger model to optimize gas cooler design over a wide range of operating conditions, eliminating the need for expensive prototype development and testing.     

Water-coupled carbon dioxide microchannel gas cooler for heat pump water heaters: Part II - Model development and validation

An experimental and analytical study on the performance of a compact, microchannel water-carbon dioxide (CO₂) gas cooler was conducted. The experimental results addressed in Part I of this study are used here in Part II to develop an analytical model, utilizing a segmented approach to account for the steep gradients in the thermodynamic and transport properties of supercritical CO₂. The model predicted gas cooler heat duty with an average absolute deviation of 7.5% with varying refrigerant and water inlet conditions. The segmented model reveals that near the pseudo-critical point, there is a significant local decrease in refrigerant-side thermal resistance, which yields a sharp increase in local heat duty. The impact of this spike on gas cooler performance is analyzed. Results from this study can be used to predict the effect of changing geometric parameters of the heat exchanger without the need for expensive prototype development and testing.     

国内外吸收式热泵强化传热传质研究综述

综述国内外对吸收式热泵强化传热传质研究的现状。目前主要的研究方向为新型强化管的开发、新型表面活性剂及强化吸收机制的研究,主要研究目的是如何增大传热面积与加强界面马拉格尼对流,以此提高传热传质系数。     

Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers

A coupled heat and mass transfer model is developed to extract the transfer coefficients for falling-films from the measurements on a tubular absorber. The mass transfer coefficients obtained from the coupled model and the log-mean-difference approach agree within about 10%. For the heat transfer coefficient, the values given by the two models can differ quite significantly. The cooling water temperature distribution predicted by the coupled model agrees well with measurements. The transfer coefficients obtained from experimental measurements using the various methods reported in the literature show wide variations.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

Experimental investigation of adverse effect of frost formation on microchannel evaporators,part 1: Effect of fin geometry and environmental effects

This study experimentally investigated the frost growth on louvered folded fins in microchannel heat exchangers when used in outdoor air-source heat pump systems. The effects of surface temperature, fin geometries, and air environmental conditions were studied. The overall aim was to isolate and quantify the effect of geometry from surface temperature effects. Experimental data of frost weight, local frost thickness, air pressure drop across the coils, time of frost–defrost cycles and heat transfer rates were recorded. Data showed that the frosting time and the frost growth rates depended mainly on the local fin surface temperature. Lower fin density was beneficial because it delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. The air humidity had a fairly significant effect on rate of frost formation while air velocity seemed to have a small effect on the frost growth rate.     

Short-time heat impulse intensification of heat transfer in liquid nitrogen

The cryogenic technology deals with fluids produced from gases after liquefaction. Boiling of cryogenic fluids is frequently characterized by a hysteresis of their boiling curve. The present experimental work demonstrates an opportunity to intensify heat transfer in those fluids by means of short-term heat impulsion from a heater. The intensification takes place due to the impulse-induced transition of heat transfer regime from natural convection towards nucleate boiling. The process takes place when the impulse magnitude overcomes certain minimum value that was quantified experimentally. We also propose a theoretical expression for the minimum energy that is in agreement with the experimental data.     

Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor

The present study reveals some of the important parameters which control the in-flight nitridation of molybdenum disilicide (MoSi₂) powders when carried out in an induction thermal plasma reactor. Initially, gradients of temperature, velocity and concentration were evaluated, using an enthalpy probe system, for the plasma flow without injection of MoSi₂ powders. Radial profiles were then measured at the torch exit to examine the mass and energy transfer mechanisms occurring under different nitridation conditions. These measurements were performed using an induction plasma torch connected to a 50 kW radio-frequency (r.f.) power supply, the torch being attached to a water cooled cylindrical reactor. The process operating conditions studied were plasma plate power, chamber pressure, sheath gas composition, composition and flow rate of quench gas. The effect of last named parameter on the nitridation of the powders was found to be the most important parameter in the nitridation process. The results show that there is an optimum flow rate value for each type of quench gas and the temperature and concentration mapping demonstrates that the combination of high temperatures and high concentrations of N₂ are necessary to reach maximum nitridation levels in MoSi₂.     

A coarse-grained parcel method for heat and mass transfer simulations of spray coating processes

The Discrete Element Method (DEM) is commonly used for modeling the flow of particulate materials. Unfortunately, such detailed simulations are computationally very demanding, restricting its use for industrially-scaled processes. The number of particles in a simulation can be reduced by introducing parcels (i.e., “coarse graining”), which – in essence – relies on the increase of the particle diameter for interaction calculations. However, sophisticated models are necessary to preserve the original behavior of the material when using such an approach. Our present contribution extends available coarse-graining concepts by introducing models for (i) particle–fluid mass transfer and (ii) the deposition rate of spray droplets on particles. Our mass transfer model is based on an existing model for heat transfer. For the spray deposition model, we introduce an effective particle diameter to compute the correct amount of droplets that impact particles. We show that these models can be used with confidence up to a coarse-graining level of 5, which we demonstrate for a simple-shaped fluidized bed. The models proposed by us are critical for detailed simulations of spray coating processes since they enable precise particle-droplet-air interaction modeling at low computational cost.     

Unsteady MHD heat and mass transfer of a non-Newtonian nanofluid flow of a two-phase model over a permeable stretching wall with heat generation/absorption

The combined effects of magnetic field and heat generation or absorption on unsteady boundary-layer convective heat and mass transfer of a non-Newtonian nanofluid over a permeable stretching wall have been addressed. A power-law model includes Brownian motion and thermophoresis influences are utilized for non-Newtonian nanofluids with a convective boundary condition. The non-linear governing equations are reduced into ODEs by similarity transformations and solved numerically by using Runge-Kutta-Fehlberg 4th–5th order numerical method (RKF45) with shooting technique. The different physical parameters effects such as the magnetic parameter $(M)$, the heat source/sink parameters $(\lambda )$, the unsteadiness parameter $(A)$, the generalized Prandtl and Lewis numbers on the dimensionless velocity, temperature and nanoparticles volume fraction, in addition to the skin friction, local Nusselt and Sherwood numbers are analyzed. It is reached that the thermal and concentration boundary-layer thickness has higher values with the increasing of magnetic field and heat generation in the case of a pseudo-plastic nanofluid than others.