Flow Boiling Heat Transfer in Coated and Uncoated Plate Heat Exchangers

The research community is currently focused on developing next generation technological solutions to utilise the energy available in domestic and industrial waste. The recoverable energy could be significant; for example, a recent study in the UK [1] identified 48 TWh/year of recoverable waste heat (17% of all industrial energy use). Organic Rankine Cycles (ORC) are best suited to harness this waste heat as they convert low and medium grade waste heat to power (<100 ℃ to 300 - 400 ℃). It is noteworthy that the potential work output and cycle efficiency, typically 16% [2], are limited by the efficiencies of the heat exchangers. Therefore improving the performance of heat exchangers in ORCs could make the utilisation of waste heat a viable concept. Plate heat exchangers (PHEs) are widely used in numerous industrial processes owing to their compact size, high surface area to volume ratio and consequently improved thermal performance, thus making them key possible component in ORCs. Although there has been numerous experimental work on the boiling of refrigerants inside PHEs, there is still a great challenge in overcoming the overall interfacial thermal-resistances between the plates that could result in significant improvements in the overall heat transfer coefficient. Boiling heat transfer can be significantly enhanced by modifying the surface characteristics. This passive enhancement technique has been investigated and applied widely in for example, shell and tube heat exchangers and less in plate heat exchangers. The performance of a brazed PHE with water as the single phase heat source and R245fa in single phase and in flow boiling mode on the other side was studied. The refrigerant-side surface was then coated and the experimental results were compared with the as-supplied heat exchanger in terms of heat transfer rates and pressure drop. The nanoFLUX  coating is a proprietary coating consisting of a metallic dendritic nano and micro structure allowing for optimisable porosity for refrigerant bubble nucleation. The results were also compared with the small number of correlations predicting the heat transfer rates on the refrigerant side and the overall heat transfer coefficient of the heat exchangers. Final recommendations for design including the possible enhancement due to the coating will be presented

Flow boiling in plain and porous coated microchannels

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 kg/m2s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 160.7 kW/m2. HFE-7200 was used as the working fluid. Flow visualisation results and SEM surface analyses are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide, while shallow but larger cavities up to 6 µm were found on the plain copper channels. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern evolution occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux. Microchannel flow boiling heat transfer is enhanced by up to 43.5 % at low heat fluxes where the nucleate boiling mechanism is dominant. Heat transfer enhancement diminishes with further increase in heat flux to 13.2 %, potentially due to nucleate boiling suppression with flow regime transition.TMD Technologies Ltd

Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly

Flow Boiling Characteristics in Plain and Porous Coated Microchannel Heat Sinks

The experimental facility was built with support from EPSRC grant EP/D500095/1

Experiments and correlations for single-phase convective heat transfer in brazed plate heat exchangers

Copyright © 2022 The Author(s). This study presents the single-phase heat transfer and pressure drop characteristics of R1233zd(e) in a Brazed Plate Heat Exchanger (BPHE). Experiments on single-phase, water-to-water, were initially conducted and a correlation for the convective heat transfer coefficient of the hot water side was derived by applying the modified Wilson plot method. The experiments covered a range of Reynolds number from 80 to 1600 and Prandtl number from 2.8 to 7.0. Subsequent experiments were conducted with water-to-R1233zd(e) covering a refrigerant range of Reynolds number from 700 to 1450 and Prandtl number from 4.5 to 4.9. The results were used to assess existing correlations in the literature predicting the Nusselt number and Fanning friction factor in BPHEs. Finally, new correlations for both the hot (water) and cold (refrigerant) sides are proposed for single-phase heat transfer for this geometry covering the conditions above. The proposed refrigerant heat transfer correlation predicted 97% of all data within the ± 10% error bands at a mean absolute error value of 5.7%.EPSRC Grant EP/P004709/1

Effects of tungsten addition on the microstructure and mechanical properties of microalloyed forging steels

In the current study, the effects of tungsten (W) addition on the microstructure, hardness, and room/low [223 K and 173 K (-50 C and -100 C)] temperature tensile properties of microalloyed forging steels were systematically investigated. Four kinds of steel specimens were produced by varying the W additions (0, 0.1, 0.5, and 1 wt pct). The microstructure showed that the addition of W does not have any noticeable effect on the amount of precipitates. The precipitates in W-containing steels were all rich in W, and the W concentration in the precipitates increased with the increasing W content. The mean sizes of both austenite grains and precipitates decreased with the increasing W content. When the W content was equal to or less than 0.5 pct, the addition of W favored the formation of allotriomorphic ferrite, which subsequently promoted the development of acicular ferrite in the microalloyed forging steels. The results of mechanical tests indicated that W plays an important role in increasing the hardness and tensile strength. When the testing temperature was decreased, the tensile strength showed an increasing trend. Both the yield strength and the ultimate tensile strength obeyed an Arrhenius type of relation with respect to temperature. When the temperature was decreased from 223 K to 173 K (from -50 C to -100 C), a ductile-to-brittle transition (DBT) of the specimen with 1 pct W occurred. The addition of W favored a higher DBT temperature. From the microstructural and mechanical test results, it is demonstrated that the addition of 0.5 pct W results in the best combination of excellent room/low-temperature tensile strength and ductility. 2013 The Minerals, Metals & Materials Society and ASM International