Natural Convection Heat Exchangers Research Articles

Unsteady Conduction in a Horizontal Solid Cylinder Cooled by Natural Convection: Alternate Lumped Criterion in Terms of the Solid Material

Within the framework of the potent lumped model, unsteady heat conduction takes place in a solid body whose space–mean temperature varies with time. Conceptually, the lumped model subscribes to the notion that the external convective resistance at the body surface dominates the internal conductive resistance inside the body. For forced convection heat exchange between a solid body and a neighboring fluid, the criterion entails to the lumped Biot number Bil=(h¯/ks)(V/A)<0.1, in which the mean convective coefficient h¯ depends on the impressed fluid velocity. However, for natural convection heat exchange between a solid body and a fluid, the mean convective coefficient h¯ depends on the solid-to-fluid temperature difference. As a consequence, the lumped Biot number must be modified to read Bil=(h¯max/ks)(V/A)<0.1, wherein h¯max occurs at the initial temperature Ti for cooling or at a future temperature Tfut for heating. In this paper, the equivalence of the lumped Biot number criterion is deduced from the standpoint of the solid thermal conductivity through the solid-to-fluid thermal conductivity ratio.

Thermal behaviour of a modular storage system when subjected to variable charge and discharge sequences

This paper presents the results of an experimental study conducted on a multi-tank thermal storage. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to standard draw profiles. The experimental setup consisted of a charge loop to simulate a solar collector input, three commercially available 270L domestic hot water tanks and three side-arm, natural convection heat exchangers. The system was charged using two 3kW electric heaters which produced a variable input power to simulate the variation in output of a typical fixed orientation solar collector. Realistic hot water draws were conducted according to the specifications of the Canadian Standard Association, CSA-F379.1, for Solar Domestic Hot Water Systems. Experimental tests were conducted over 2days for three different plumbing configurations, and compared with simulation results produced in the TRNSYS simulation environment. The plumbing configurations which were considered include: series charge and series discharge, parallel charge and parallel discharge, and series charge and parallel discharge. Comparisons between system configurations were based on stratification levels, hot water delivery temperatures, and solar fractions. Results of the study showed that charging and discharging the system in parallel produced the highest solar fraction when compared to the other multi-tank configurations, with solar fractions of 0.62 and 0.67 from experimental testing and TRNSYS simulation, respectively. When compared to a single tank configuration, the parallel charge and parallel discharge configuration achieved 97.7% of the delivered solar energy of a stratified single tank system with the same storage volume and heat exchanger performance.

Charge and discharge strategies for a multi-tank thermal energy storage

This paper presents the results of an experimental study conducted on a multi-tank thermal storage for solar hot water heating applications. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to constant temperature charging and constant volume hourly draws. The experimental setup consisted of a charge loop to simulate a solar collector input, three commercially available 270L domestic hot water tanks and three side-arm, natural convection heat exchangers. Tests were performed over 8h periods for three different plumbing configurations and two different hourly draw volumes. Simulations were conducted using the TRNSYS simulation environment, and the results showed that the TRNSYS model was in good agreement with the experimental results, where the discrepancies between data were found mainly in the regions of high temperature gradients within the storage tanks. Preliminary simulations were also conducted using realistic charge and draw profiles to illustrate the thermal behaviour of the tanks under non-ideal operating conditions.

Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, CR=2 and a 100mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.

The Effect of Discharge Configurations on the Thermal Behaviour of a Multi-tank Storage System

This paper presents the results of an experimental study conducted on a multi-tank thermal storage. The purpose of the study was to investigate the thermal behaviour of the stratified tank system when subjected to standard draw profiles. The experimental set up consisted of a charge loop to simulate a solar collector input, three commercially available 270 L domestic hot water tanks and three side-arm, natural convection heat exchangers. The system was charged using two 3kW electric heaters which produced a variable input power to simulate the variation in output of a typical fixed orientation solar collector. Realistic hot water draws were conducted according to the specifications of the Canadian Standard Association, CSA-F379.1, for Solar Domestic Hot Water Systems. Tests were performed over 2 days for three different plumbing configurations, and compared with simulated results produced in the TRNSYS simulation environment. Comparisons between system configurations were based on stratification levels and hot water delivery temperatures.

Numerical investigation on the use of nanofluids in natural convection heat exchangers using a mixture-phase approach

In the present research, the behavior of a Newtonian nanofluid (water–Al2O3) in a mixture phase model approach is numerically examined. The process of heating is done in two different ways. Deterioration was found in the mean Nusselt number of a nanofluid in the mixture-phase model approach when compared to the mean Nusselt number of pure water. However, in the single-phase model there was an increase in the Nusselt number when compared to the Nusselt number of pure water. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20383

The influence of concentrated solar radiation on the properties of Ga0.7In0.3P/GaP photoconverters

The properties of Ga0.7In0.3P/GaP heterophotoconverters with a two-sided contact grid are studied in the range of solar radiation concentration Ks = 1−100 times and with natural convective heat exchange. It is found that, in the considered photoconverters, the dependences of an idle running voltage, the duty cycle of the current-voltage characteristic, and the efficiency on concentration are additionally improved due to high heat conduction of GaP and the temperature stability of the broad-band heterophotoconverter with “transparent“ structural design.

Investigation of Reverse Thermosyphoning in an Indirect SDHW System

Thermal energy storages with thermosyphon natural convection heat exchangers have been used in solar water heating systems as a means of increasing tank stratification and eliminating the need for a second circulation pump. However, if the storage system is not carefully designed, under adverse pressure conditions, reverse thermosyphoning can result in increased thermal losses from the storage and reduced thermal performance of the system. To investigate this phenomenon, tests were conducted on single tank and multitank thermal storages under controlled laboratory conditions. Energy storage rates and temperature profiles were experimentally measured during charge periods, and the effects of reverse thermosyphoning were quantified. Further objectives of this study were to empirically derive performance characteristics, to develop numerical models to predict the performance of the heat exchanger during reverse thermosyphon operation, and to quantify the relative magnitude of these effects on the energy stored during typical daylong charge periods. Results of this study show that the magnitude of the reverse flow rate depends on the pressure drop characteristics of the heat exchange loop, the system temperatures, and the geometry of the heat exchanger and storage tank. In addition, the results show that in the case of a multitank thermal storage, the carryover of energy to the downstream thermal energy storages depends on the effectiveness of the exchangers used in the system.

Thermal response of a series- and parallel-connected solar energy storage to multi-day charge sequences

The thermal response of a multi-tank thermal storage was studied under variable charge conditions. Tests were conducted on an experimental apparatus that simulated the thermal charging of the storage system by a solar collector over predetermined (prescribed) daylong periods. The storage was assembled from three standard 270 L hot-water storage tanks each charged through coupled, side-arm, natural convection heat exchangers which were connected in either a series- or parallel-flow configuration. Both energy storage rates and tank temperature profiles were experimentally measured during charge periods representative of two consecutive clear days or combinations of a clear and overcast day. During this time, no draw-offs were conducted. Of particular interest was the effect of rising and falling charge-loop temperatures and collector-loop flow rate on storage tank stratification levels. Results of this study show that the series-connected thermal storage reached high levels of temperature stratification in the storage tanks during periods of rising charge temperatures and also limited destratification during periods of falling charge temperature. This feature is a consequence of the series-connected configuration that allowed sequential stratification to occur in the component tanks and energy to be distributed according to temperature level. This effect was not observed in the parallel charge configuration. A further aspect of the study investigated the effect of increasing charge-loop flow rate on the temperature distribution within the series-connected storage and showed that, at high flow rates, the temperature distributions were found to be similar to those obtained during parallel charging. A disadvantage of both the high-flow series-connected and parallel-connected multi-tank storage is that falling charge-loop temperatures, which normally occur in the afternoon, tend to mix and destratify the storage tanks.

On Liquid-Nitrogen-Cooled Copper-Wound Machines With Soft Magnetic Composite Core

This paper illustrates the potential and the convenience of copper-wound electrical machines with liquid nitrogen (LN) cooling. Natural convection heat exchange coefficients in LN are very high. Moreover, the resistivity of copper decreases remarkably at cryogenic temperature, subsequently reducing Joule loss in copper windings. Hence, cryogenic copper-wound electrical machines use less copper to carry the same current compared to airor water-cooled machines, thereby improving the powerto-weight ratio. An experimental study is made of some practical issues related to the design of an LN-cooled machine using as an example the construction of a 220-N permanent magnet tubular linear actuator with a soft magnetic composite stator core.

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

The high capital costs associated with heat-pipe evacuated tube solar water heating systems can be reduced by replacing forced circulation with thermosyphon circulation. Currently research on thermosyphon heat-pipe evacuated tube solar water heaters is limited. An experimental investigation of the natural convective heat exchange regime that exists within the manifold chamber of a proprietary heat-pipe evacuated tube solar water was undertaken. This paper presents experimental data from a heat-pipe Evacuated Tube Solar Water Heater (ETSWH) subjected to the Northern Maritime Climate at the University of Ulster’s outdoor solar testing facility located at the Jordanstown campus. The thermal performance of this across solar noon (±30 min) was experimentally determined to be comparable to two physical laboratory 10 pin-fin model manifolds constructed to the same dimensions and geometry as the manifold chamber of the heat-pipe ETSWH when operated under steady laboratory conditions. When the surface temperatures of the pin-fins (simulated condensers) in the model manifold were normalised with respect to the lowest most pin-fin in the array the influence of buoyant flow was observed. Similarly to related studies in this field it was found that normalised surface temperatures on downstream pin-fins do not increase monotonically as would be expected if no interactions occur. It was found that at the pin-fin diameter to pitch used in the model manifold that normalised surface temperatures decrease at certain points in the array due to the action of buoyant flow generated from upstream pin-fins which increased heat transfer. Two-dimensional Particle Imaging Velocimetry (2D-PIV) was used to visualise the thermosyphon fluid flow regime. It was observed that the fluid flow regime varied across the model due to interactions between the fluid, chamber walls and pin-fins.

Thermal modelling of a night sky radiation cooling system

The thermal modelling of a night sky radiation cooling system suitable for a room situated in the Namib Desert at Gobabeb, Namibia, is considered in this paper. The system consists of the following components: radiator panels, a single water storage tank, room air-to-water natural convection heat exchangers or convectors, circulating pump(s), interconnecting pipe work and temperature sensors and controls. The mathematical equations describing the thermal behaviour of the various system components are given. These equations are solved using an Excel spreadsheet and the hourly panel surface, water storage tank and room temperatures are calculated for a given internally generated heat load and weather pattern. Given the maximum allowable room temperature, the sizes of the system components may be calculated. The results obtained compared favourably with values reported in the literature. It is thus concluded that the thermal model presented can be used with confidence as a design tool for the sizing of a night sky radiation cooling system.

DESIGN PROCEDURE FOR SELECTING AN OPTIMUM PHOTOVOLTAIC PUMPING SYSTEM IN A SOLAR DOMESTIC HOT WATER SYSTEM

A search methodology for the optimum configuration of a photovoltaic pumping system in a solar domestic hot water (PV-SDHW) system is presented. The main premise of this methodology is the decoupling of the optimum search process in two phases. In phase one the goal is to find, among the possible PV pumping systems' flow rate profiles, the profile that maximizes the performance of a given SDHW system. In phase two the goal is to select the components of the PV pumping system. Each combination of PV cells, DC-motor and pump exhibits a unique flow rate profile. Therefore, the problem in this phase is to identify the PV pumping system components that results in the best match to the optimum flow rate profile found in phase one. This decoupling considerably eased the process of optimal search. An analysis of actual flow rate profiles of PV pumping systems showed that the optimum search can be restricted to the family of profiles represented by an experimentally-validated generalized function dependent upon three parameters only. Three PV-SDHW systems were considered and the optimum search methodology applied. The three systems are: (i) direct PV-SDHW system employing a PV array, a DC-motor, a rotary pump, a thermal collector, and a storage tank, (ii) and (iii) indirect PV-SDHW systems employing similar components to those in the direct PV-SDHW system in addition to either a forced or a natural convection heat exchanger to transfer the energy from the collector-side fluid to the tank-side fluid. Three DC-motors were considered; (i) series excited, (ii) permanent magnet, and (iii) separately excited.

Thermal Performance of Metallic and Non-Metallic Natural Convection Heat-Exchanger Fins

The steady-state rates of heat dissipation from an array of vertical rectangular heatexchanger fins to the ambient air had been measured experimentally under natural convection conditions, when their horizontal base was maintained at an uniform temperature of 40 K above that of the ambient environment which was kept uniformly at 20°C The fins were 3 mm thick and 500 mm long, and protruded 60 mm perpendicularly from the 500 mm X 190 mm horizontal rectangular base. The fins were fabricated with duralumin ( kf = 160 Wm−1 K−1), stainless steel (kf = 15 Wm−1 K−1) and fibre plastic (kf = 0.2 Wm−1K−1) respectively. The optimal fin separation corresponding to the maximum rate of heat loss from the fin array was increased from 14.5 mm to 18 mm, and the maximum rate of heat dissipation was only slightly reduced by 12% when stainless steel fins were used instead of duralumin fins. There was no optimal fin separation observed when fibre plastic was used to fabricate the fin array. and the maximum heat dissipation rat...

Simulation of the heat transfer of a lithospheric platform in the subduction zone. II. Method of solution and results of calculations

Based on the control volume method, a numerical algorithm is developed for solving the problem of natural convective heat exchange in the earth's upper mantle.

96/01742 An empirical model for natural convection heat exchangers in SDHW systems

96/01742 An empirical model for natural convection heat exchangers in SDHW systems

An empirical model for natural convection heat exchangers in SDHW systems

Natural convection heat exchangers are valuable devices in solar domestic hot water systems, but they are difficult to model. The output pressures and temperatures they develop are complex functions of the flow and temperatures that they are subjected to, and these latter quantities—which vary over each hour in the year—are themselves dependent on the exchanger outputs. Yet to optimize the exchanger and to be truly confident of its performance, one needs good models which can be embedded into total simulation codes, like WATSUN or TRNSYS. This paper gives a suitable exchanger model and outlines how it can be used to model a total system. The model calls for a set of steady state experiments to be carried out on the exchanger in question, characterizing the exchanger. The experiments produce curves of collector effectiveness and shear pressure drop, both plotted against flow rate. A suitable experimental apparatus is described, and the results obtained for a particular exchanger are reported. Also reported are transient experiments that verify the applicability of the steady state experiments to real, time-varying conditions.

An empirical model for natural convection heat exchangers in SDHW systems

Natural convection heat exchangers are valuable devices in solar domestic hot water systems, but they are difficult to model. The output pressures and temperatures they develop are complex functions of the flow and temperatures that they are subjected to, and these latter quantities—which vary over each hour in the year—are themselves dependent on the exchanger outputs. Yet to optimize the exchanger and to be truly confident of its performance, one needs good models which can be embedded into total simulation codes, like WATSUN or TRNSYS. This paper gives a suitable exchanger model and outlines how it can be used to model a total system. The model calls for a set of steady state experiments to be carried out on the exchanger in question, characterizing the exchanger. The experiments produce curves of collector effectiveness and shear pressure drop, both plotted against flow rate. A suitable experimental apparatus is described, and the results obtained for a particular exchanger are reported. Also reported are transient experiments that verify the applicability of the steady state experiments to real, time-varying conditions.

Steady state model for the thermal regimes of shells of airships and hot air balloons

A steady state model of the temperature regime of airships and hot air balloons shells is developed. The model includes three governing equations: the equation of the temperature field of airships or balloons shell, the integral equation for the radiative fluxes on the internal surface of the shell, and the integral equation for the natural convective heat exchange between the shell and the internal gas. In the model the following radiative fluxes on the shell external surface are considered: the direct and the earth reflected solar radiation, the diffuse solar radiation, the infrared radiation of the earth surface and that of the atmosphere. For the calculations of the infrared external radiation the model of the plane layer of the atmosphere is used. The convective heat transfer on the external surface of the shell is considered for the cases of the forced and the natural convection. To solve the mentioned set of the equations the numerical iterative procedure is developed. The model and the numerical procedure are used for the simulation study of the temperature fields of an airship shell under the forced and the natural convective heat transfer.

Natural convection heat exchangers in solar water heating systems: Theory and experiment

Natural convection heat exchangers can be used in solar hot water systems to replace the pump on the tank side of the exchanger. There is currently no experimentally verified way of designing this type of heat exchanger. An experimental apparatus to test natural convection heat exchangers was built and an extensive set of measured data obtained on two different exchangers sized for low-flow stratified tank system. Two theoretical models for the exchanger are presented: a finite-volume primitive variable numerical solution of the fundamental laminar equations of fluid motion and a laminar forced-convection-based solution method. Comparison of the model predictions with the experimental data showed good agreement when the modified Rayleigh number is less than about 400. The poor agreement under other conditions was attributed to turbulence and recirculation neither of which was accounted for in the models.