Michael R. Collins

NUMERICAL ANALYSIS OF HEAT TRANSFER IN A SQUARE CAVITY WITH A BAFFLE ON THE HOT WALL

A finite-volume based numerical analysis is performed to investigate the effect of attaching a high conducting thin baffle on the hot wall of a square cavity. Accordingly, a mathematical model is developed. The present work is intended to study the enhancement of laminar natural convection from a vertical hot wall with baffle attached on the surface, where the horizontal walls are kept insulated. The effects of baffle height, length, and Rayleigh number on heat transfer performance are investigated numerically for air as the working fluid

Numerical and experimental study of heat transfer in a BIPV-thermal system

This paper presents a computational fluid dynamics (CFD) study of a building-integrated photovoltaic thermal (BIPV/T) system, which generates both electricity and thermal energy. The heat transfer in the BIPV/T system cavity is studied with a two-dimensional CFD model. The realizable k-e model is used to simulate the turbulent flow and convective heat transfer in the cavity, including buoyancy effect and long-wave radiation between boundary surfaces is also modeled. A particle image velocimetry (PIV) system is employed to study the fluid flow in the BIPV/T cavity and provide partial validation for the CFD model. Average and local convective heat transfer coefficients are generated with the CFD model using measured temperature profile as boundary condition. Cavity temperature profiles are calculated and compared to the experimental data for different conditions and good agreement is obtained. Correlations of convective heat transfer coefficients are generated for the cavity surfaces; these coefficients are necessary for the design and analysis of BIPV/T systems with lumped parameter models. Local heat transfer coefficients, such as those presented, are necessary for prediction of temperature distributions in BIPV panels.

EVALUATION OF AN APPROXIMATE METHOD FOR PREDICTING THE U VALUE OF A WINDOW WITH A BETWEEN-PANES BLIND

A numerical model has been developed of the conjugate convection, conduction, and radiation heat transfer in a double-glazed window with a between-panes venetian-type blind. The two-dimensional finite-volume model has been validated for a range of slat angles using U value measurements from the literature. Data from this solution have been used to show that the thermal performance of the window/blind enclosure can be closely approximated using a simplified model. Such simplified methods of analysis are of utility in fenestration design software.

Heat Transfer From an Isothermal Vertical Surface With Adjacent Heated Horizontal Louvers: Validation

The present study examines the influence of heated, horizontal, and rotateable louvers on the convective heat transfer from a heated or cooled vertical isothermal surface. The system represents an irradiated Venetian blind adjacent to the indoor surface of a window. Detailed temperature field and local surface flux data were obtained using a Mach-Zehnder Interferometer for two window temperatures (warm and cool compared to ambient) and irradiation levels, two louver to plate spacings, and three louver angles. The results have been compared to a steady, laminar, two-dimensional, conjugate conduction/ convection/radiation finite element model of this problem

The Effect of Coverings on Heat Transfer from a Window to a Room

Abstract The presence of a blind adjacent to a window affects the natural convective and radiant heat transfer from the window to the room. As a result, the use of a shading device will change the heat transmission and solar heat gain through the window. A number of numerical and experimental studies of the effects of blinds on the heat transfer from a window have therefore been undertaken, with some of the main features of these studies being described here. In these studies, attention has been given to Venetian, vertical, and plane blinds, although the major attention has been given to Venetian blinds. Initial studies examined the effect of all three types of blinds on the natural convective heat transfer at an indoor glazing surface when there is no solar irradiance. Supporting experimental studies using mainly interferometry were then undertaken, particularly for the Venetian blind case. The numerical and experimental work was then extended to include the effects of solar radiation, in particular the effect of heat generation in the blind resulting from absorbed solar radiation. In addition to providing basic information on the effects of blinds on the heat transfer process, the studies described here will assist in expanding available software for predicting window heat transfer to include the effect of window coverings and assist in the selection of energy-efficient window coverings.

Heat Transfer From an Isothermal Vertical Surface With Adjacent Heated Horizontal Louvers: Numerical Analysis

The present study examines the influence of heated, horizontal, and rotatable louvers on the convective and radiative heat transfer from a heated or cooled vertical isothermal surface. The system represents an irradiated Venetian blind adjacent to the indoor surface of a window. Detailed heat transfer results were obtained using a steady, laminar, two-dimensional, conjugate conduction/convection/radiation finite element model for two window temperatures (warm and cool compared to ambient) and irradiation levels, two louver to surface spacings, and three louver angles. The effect of the heated louvers on the heat transfer rate from the surface has been demonstrated

Characteristic effectiveness curves for falling-film drain water heat recovery systems

Falling-film drain water heat recovery systems have proven to be a cost-effective and reliable class of heat exchanger for reducing primary energy consumption in residential and commercial buildings and in industrial buildings and processes. It is fitting, therefore, that regulatory bodies are preparing standards by which various products can be characterized, both for rating purposes and to provide data for building energy simulation. Unfortunately, standards development is progressing in the absence of measured performance data that characterize how these heat exchangers perform. The intention of the current work is to examine drain water heat recovery performance at various equal-flow conditions. The effectiveness of three drain water heat recovery systems was examined in a counter flow as a function of volumetric flow rate. The drain water heat recovery systems represented products from two manufacturers and two lengths. One of the drain water heat recovery systems was also tested in parallel flow. While the performance characteristics generally mirrored theoretical performance, there were some key differences. For the units tested, there was a clear transition region occurring between flow rates of 5 and 10 L/min (1.3 and 2.6 gpm). While the presence of this region did not impact the proposed rating process, it could significantly impact the applicability of the data analysis to building simulation. It was also shown that the number of transfer units for the collector changed significantly with flow rate, but in a predictable manner. By fitting the number of transfer units versus flow rate data, and using this correlation in conjunction with theoretical ϵ–number of transfer units equations, the drain water heat recovery performance could be well predicted over the entire range of operation.

Geometric Optimization of Radiant Enclosures Containing Specularly-Reflecting Surfaces through Quasi-Monte Carlo Simulation

Monte Carlo (MC) ray-tracing simulation coupled with Kiefer–Wolfowitz stochastic programming is a standard tool for designing enclosures that contain specularly-reflecting surfaces. Unfortunately, the statistical uncertainty MC induces into the objective function combined with the error amplification inherent in finite-difference approximations of the gradient means that large numbers of bundles are often needed to obtain acceptable search directions. This article shows that quasi-Monte Carlo sampling reduces these uncertainties, allowing the required gradient estimations to be carried out using smaller sample sizes, in turn leading to a dramatic drop in optimization time.

Experimental and numerical results for a building-integrated photovoltaics test facility

The results of an experimental and simulation study of a double facade with integrated photovoltaic panels are presented and analyzed. The system consists of a double facade with integrated photovoltaic panels at the bottom part. Air enters from the bottom through an intake, gets heated as it flows upwards in the cavity, driven by a fan and buoyancy, and finally enters the HVAC system as preheated fresh air. Experimental and numerical results show that a combined thermal-electric efficiency of the system could exceed 70%. CFD simulations are employed to determine accurately heat transfer coefficients needed for energy calculations.

Thermal energy harvesting between the air/water interface for powering wireless sensor nodes

Seventy percent of the Earths surface is covered by water and all living things are dependent upon this resource. As such there are many applications for monitoring environmental data in and around aquatic environments. Wireless sensor networks are poised to revolutionise this process as the reduction in size and power consumption of electronics are opening up many new possibilities for these networks. Aquatic sensor nodes are usually battery powered, so as sensor networks increase in number and size, replacement of depleted batteries becomes time consuming, wasteful and in some cases unfeasible. Additionally, a battery that is large enough to last the life of a sensor node would dominate the overall size of the node, and thus would not be very attractive or practical. As a result, there is a clear need to explore novel alternatives to power sensor nodes/networks, as existing battery technology hinders the widespread deployment of these networks. By harvesting energy from their local environment, sensor networks can achieve much greater run-times, years not months, with potentially lower cost and weight. A potential renewable energy source in aquatic environments exists via the temperature gradient present between the water layer and ambient air. A body of water will be either a few degrees warmer or colder than the air directly above it dependant on its latitude, time of year and time of day. By incorporating a thermal energy harvesting device into the sensor node deployment which promotes the flow of heat energy across the thermal gradient, a portion of the energy flow can be converted into useable power for the sensor node. To further increase this temperature difference during the day the top section can be heated to temperatures above the ambient air temperature by absorbing the incoming sunlight. As an initial exploration into the potential of this novel power source we have developed a model of the process. By inputting environmental data, the model calculates the power which can be extracted by a thermal energy harvesting device. Initial outputs show a possibility of up to 10W/m2 of power available from measured sites assuming a thermal energy harvester operating with Carnot efficiency.