Heat transfer in low reynolds number flows through miniaturised channels

The use of miniaturized channels as heat sinks/ heat exchangers is of great importance due to advantages of compact size and high heat sinking capabilities. The small hydraulic diameters of these miniaturized channels imply a higher pressure drop and consequently higher pumping power. Therefore, where possible, the pumping power required may be reduced by running applications at low flow velocities. This paper therefore investigates the heat transfer in low Reynolds number flow through miniaturized channels. Forced convection experiments were performed on 2 instrumented metal plates with 0.5 mm and 0.25 mm deep channels respectively. The channels were 2 mm wide and 270 mm long. A propylene glycol-based heat transfer fluid for solar collectors, Tyfocor® LS, was used as the working fluid. Reynolds numbers were in the range 10 – 100 and fluid inlet temperatures ranged from 5 – 60 °C. The measured Nusselt numbers were observed to depend more on the Peclet number and less on the geometry. Peclet number dependent Nusselt numbers was attributed to miniaturization scaling effects. A correlation for estimating the Nusselt number in terms of the Peclet number and hydraulic diameter was proposed. The results are significant in predicting the heat sinking performance in applications having low Reynolds number flows through miniaturized channels such as compact solar thermal collectors.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Design and commissioning of a virtual image solar simulator for testing thermal collectors

A solar simulator has been designed and built for testing prototype (0.5×0.5 m) flat plate thermal collectors. An internally reflecting light tube generates multiple virtual images of the four halogen floodlights to ensure uniform illumination. Ray-tracing simulations were used to choose the tube dimensions and maximum allowable clearance. Illumination measurements agree well with these predictions. The visible & near IR spectrum appears to follow a black body curve. In the absence of a “cold sky” IR filter there is a secondary, long wavelength IR spectral component that causes heating of the cover glass on a solar flat plate collector. The cover glass temperature can be maintained at typical outdoor levels using a cooling fan. The design would be well suited to LED illumination. Simulation of solar collector response to this spectrum shows that an efficiency based on pyranometer readings is approximately 1% higher than would be obtained with an AM1.5 spectrum

Simulator testing of evacuated flat plate solar collectors for industrial heat and building integration

The concept of an evacuated flat plate collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This paper demonstrates the reduction in heat loss coefficient and increase in efficiency resulting from evacuating a flat plate collector: it is hoped that these results will stimulate interest in the concept. Evacuated tubes are now mass-produced in large numbers; evacuated flat plate collectors could in principle replace these tubes if the technical difficulties in creating extended metal-glass seals can be overcome. The experimental experiences described here should indicate targets for future research. Two different designs of evacuated flat plate solar thermal collector, each with a 0.5x0.5m flooded panel black chrome plated absorber, were tested under a solar simulator. The cover glasses were supported by an array of 6 mm diameter pillars. Inlet and outlet temperatures were monitored via PT100 RTDs and glass temperatures were measured using thermocouples. Inlet temperature was controlled by a fluid circulator connected to a header tank with a Coriolis mass flow meter to measure fluid flow rate. Testing was conducted indoors with and without the use of a fan to cool the top cover glass. The test conditions spanned the range 200<G<1000 W/m², 0 less than or equal to TM/G less than or equal to 52°C. Evacuating the enclosure reduced the measured heat loss coefficient by 3.7 W/m²K: this was a close match to predictions and corresponds to an increase in aperture efficiency from 0.3 to 0.6 at TM/G = 0.06 m2K/W. The poor efficiency under non-evacuated conditions was due to the black chrome absorber coating being less selective than commercial panel coatings. The solder seals were developed from experience with vacuum glazing but the increased gap led to reliability issues. A vacuum pump maintained the enclosures under a high vacuum (<0.1 Pa) during testing. The enclosure based on a thin rear metal tray proved to be more effectively sealed than the more rigid enclosure with glass on both sides: the latter developed leaks as the front to rear temperature difference increased. The biggest challenge in the manufacture of evacuated flat plate collectors is to ensure a long-term hermetic seal such that no pumping is required

Performance and operational effectiveness of evacuated flat plate solar collectors compared with conventional thermal, PVT and PV panels

The concept of an evacuated flat plate (EFP) collector was proposed over 40 years ago but, despite its professed advantages, very few manufacturers have developed commercial versions. This situation suggests both technical difficulties in manufacturing a competitively-priced sealed for life panel and a lack of awareness of the bene fits of such panels. This paper demonstrates an evacuated flat plate simulation that closely models experimental efficiency measurements. Having established the validity of the model, it compares published data for a commercial EFP collector with predictions for an optimal design to investigate whether any further efficiency improvement might be possible. The optimised design is then evaluated against alternative solar energy devices by modelling a number of possible applications. These comparisons should inform choices about solar options for delivering heat: EFP collectors are well-suited to some of these applications. Evacuated flat plate collectors are a possible alternative to concentrating collectors for Organic Rankine Cycle power generation. The annual output for all the modelled collectors was found to be a quadratic function of delivery temperature: this enabled a novel optimisation of ORC source temperature. Predictions for concentrating and non-concentrating ORC plant are compared with a PV/thermal alternative. The ORC output is significantly less than a PV panel would achieve; applications needing both heat and power are better served by PVT panels. This is an original and novel result

Characterizing Micro-channel Absorber Plates for Building Integrated Solar Thermal Collectors

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.This paper discusses the characterisation of micro-channel absorber plates for Compact Flat Plate solar thermal collectors, which are suitable for Building Integration. Experimental and computational studies were carried out at typical operating conditions for flat plate solar collectors. Three-dimensional numerical analysis using commercial CFD package, Ansys CFX, showed that heat transfer occurred on only three surfaces of the channel and there was a peripheral variation of the heat flux density. It was also observed that axial thermal conduction could modify the surface boundary at the inlet and outlet, however, the middle section of the channel could be approximated as a rectangular channel with three walls transferring heat under a H1 boundary condition. Experimental studies were used to estimate the standard parameters for predicting performance of the flat plate collectors, which indicated promising performance results. The collector flow factor F” and the heat removal factor could be improved by increasing the collector capacitance rate; this can be achieved by increasing the mass flow rate per collector area m ̇/A_c, as well as reducing the overall heat loss, UL . This analysis is important for optimizing design and operating parameters, especially to minimize temperature gradient in the transverse and longitudinal directions

Pressure drop of flowing ice slurries in industrial heat exchangers

Experiments were conducted to determine the characteristics of ice flows through industrial heat exchangers (a Tetra Plex (R) C6-SR and a Tetra Spiraflo (R) MTC70/W-3). This type of equipment presents many problems with respect to cleaning and is therefore of particular interest when considering a pigging system using ice slurry. Moderately thick ice slurries (in the range 15-60% solids) were successfully pumped through commercial heat exchangers. Measured pressure drops were greater than those with water, and rose with increasing ice fraction and flow rate. Evidence was seen for an exponential dependence of pressure drop on ice fraction, in addition to a water-like dependence on the square of flow rate. Blocking events were observed at higher ice fractions, or when large crystal masses were thought to be present in the ice slurry. It is likely that the risks of this could be mitigated by better mixing of the bulk ice slurry prior to and during delivery. For some tests the ice crystals may have grown to almost 1 mm in diameter, which is close to the characteristic dimension of the flow channels in the PHE (mean width 4 mm). increasing the likelihood of blockages forming

The anomalous pressure drop behaviour of ice slurries flowing through constrictions

The effective ‘viscosity’ for ice slurry containing more than say 30 to 40% ice fraction is many orders of magnitude higher than that for water. However, this work suggests that such slurries, when flowing through constrictions, can incur lower pressure drops than water flowing at the same rate through the same topology. This paradoxical finding is explored, studied and reported in this paper. It is thought that this reduction in pressure drop is the result of ice particles inhibiting the onset of turbulence in the ice slurry. Other researchers have previously observed this phenomenon in slurries of lower ice fractions. It is also suggested that the relatively low pressure drops observed for high ice fractions may be due to complex heat transfer and phase change phenomena, which are likely to occur at the ice-wall interface

Parametric analysis of influencing factors in Phase Change Material Wallboard (PCMW)

Incorporating Phase Change Materials (PCMs) into traditional building structures has been considered as an effective way to reduce the mismatch between energy supply and demand and in turn to minimise energy consumption (cooling/heating energy). For building applications, Phase Change Material Wallboards (PCMWs) are of particular interest due to their easy installation to existing buildings for refurbishment. Both interior and exterior PCMWs are investigated in this paper, with a numerical study examining the effects of wallboard thermal properties on its thermal performance. These influencing factors include melting temperature, melting range, latent heat, thermal conductivity and surface heat transfer coefficient. An effective heat capacity model is adopted to consider latent heat with the model validated by an experiment. Inner surface temperature and diurnal energy storage are chosen as the evaluation criteria when comparing the thermal performance between different PCMWs. By analysing the effects of influencing factors on the system thermal performance, this study serves as a useful guide for selection of PCMs in energy-efficient buildings.Peer reviewe