Performance of Plate Heat Exchangers Used as Refrigerant Liquid-Overfeed Evaporators

The heat transfer and pressure drop characteristics of plate heat exchangers were measured, when used as refrigerant liquid over-feed evaporators. The three units all had 24 plates but with different chevron-angle combinations of 28°/28°, 28°/60°, and 60°/60°. R134a flowing upwards was used as the refrigerant, in a counter-current arrangement with water flowing on the other side. Heat transfer and pressure drop measurements were made over a range of mass flux, heat flux and corresponding outlet vapour fractions. The effect of system pressure on the evaporator performance was not evaluated due to the small range of evaporating temperature. Experimental data were reduced to obtain the refrigerant-side heat transfer coefficient and frictional pressure drop. The results for heat transfer showed a strong dependence on heat flux and weak dependence on mass flux and vapour fraction. Furthermore, the chevron angle had a small influence on heat transfer but a large influence on frictional pressure drops. Along with observations that were obtained previously on large ammonia and R12 plate evaporators, it is concluded that the dominating heat transfer mechanism in this type of evaporator is nucleate-boiling rather than forced convection. For the two-phase friction factor, various established methods were evaluated; the homogeneous treatment gives good agreement.

Numerical Investigation of Heat Transfer for Laminar and Turbulent Flow in a Plate Heat Exchanger

The problem of turbulent flow and heat transfer in a two-channel plate heat exchanger was numerically investigated, considering its complex geometry as well as inlet and outlet ports effects. Results obtained for the flow and thermal field have clearly shown their asymmetrical behavior, which has important influence on the local heat transfer. Friction factor are found to be in good agreement with theoretical correlation.

Falling Film Evaporation of Water on Horizontal Configured Tube Bundles

This paper reports an experimental study on falling film evaporation of water on 6-row horizontal configured tube bundles in a vacuum. Three types of configured tubes, Turbo-CAB-19fpi and −26fpi, Korodense, including smooth tubes for reference, were tested in a range of film Reynolds number from about 10 to 110. Results show that as the falling film Reynolds number increases, falling film evaporation goes from tubes partial dryout regime to fully wet regime; the mean heat transfer coefficients reach peak values in the transition point. Turbo-CAB tubes have the best heat transfer enhancement of falling film evaporation in both regimes, but Korodense tubes’ overall performances are better when tubes are fully wet. The inlet temperature of heating water has hardly any effects on the heat transfer, but the evaporation pressure has controversial effects. A correlation with errors within 10% was also developed to predict the heat transfer enhancement capacity.

Vacuum Operation of a Thermosyphon Reboiler

The research facility at the University of Manchester in the Morton Laboratory is a full scale replica of an industrial sized natural circulation thermosyphon reboiler, which comprises 50 tubes of 3 m length and 25.4 mm OD. The facility is operated under vacuum. Water is used as the process fluid and condensing steam is the heating source. Experimental datasets were obtained for the reboiler and have been presented in the form of profile plots of feed rate, fluid recirculation, recirculation ratio and vapour quality. The data elucidate the effect of pressure [0.1 to 1.0 bar] and heat duties [78 to 930 kW] on the performance of the reboiler. Three distinct modes of operation have been observed. Mode one is defined as a flow-induced instability or geysering (low heat duty) and exists below a definite transitional point that is independent of process pressure. Mode two is a region of stable operation that occurs above the threshold of the flow-induced instability, while mode three, which is defined as the heat-induced instability (density-wave instability), is pressure dependent obtained at high duties and is characterised by violent oscillations. These instability thresholds represent the lower and upper limits of operation of the reboiler. The region of stable operation is enveloped between the two limits and is very dependent on process pressure as it progressively becomes smaller as the vacuum becomes lower. These studies led to unique experimental observations, which revealed the existence of intermittent reversed flow in the entire loop. The use of throttling in the heat-induced unstable region to return to stable operation tends to be over a narrow range, outside of which the sole way to regain stability is to lower the heat load or increase the process pressure. In the region of flow-induced instability, throttling the fluid at the inlet is useless and actually makes the situation worse. These instabilities are alleviated by increasing the heat load.

On Thermal Performance of Seawater Cooling Towers

Seawater cooling towers have been used since the 1970’s in power generation and other industries, so as to reduce the consumption of freshwater. The salts in seawater are known to create a number of operational problems including salt deposition, packing blockage, corrosion, and certain environmental impacts from salt drift and blowdown return. In addition, the salinity of seawater affects the thermophysical properties which govern the thermal performance of cooling towers, including vapor pressure, density, specific heat, viscosity, thermal conductivity and surface tension. In this paper, the thermal performance of seawater cooling towers is investigated using a detailed model of a counterflow wet cooling tower. The model takes into consideration the coupled heat and mass transfer processes and does not make any of the conventional Merkel approximations. In addition, the model incorporates the most up-to-date seawater properties in the literature. The model governing equations are solved numerically and its validity is checked by data in the literature. Based on the results of the model, a correction factor is obtained which characterizes the degradation of the cooling tower effectiveness when seawater is used.

A Planar Labyrinth Micromixer

Rapid mixing of two fluids in microchannels has posed an important challenge to the development of many integrated lab-on-a-chip systems. In this paper, we present a planar labyrinth micromixer (PLM) to achieve rapid and passive mixing by taking advantage of a synergistic combination of the Dean vortices in curved channels, a series of perturbation to the fluids from the sharp turns, and an expansion and contraction of the flow field via a circular chamber. The PLM is constructed in a single soft lithography step and the labyrinth has a footprint of 7.32 mm × 7.32 mm. Experiments using fluorescein isothiocyanate solutions and deionized water demonstrate that the design achieves fast and uniform mixing within 9.8 s to 32 ms for Reynolds numbers between 2.5 and 30. Compared to the mixing in the prevalent serpentine design, our design results in 38% and 79% improvements on the mixing efficiency at Re = 5 and Re = 30 respectively. An inverse relationship between mixing length and mass transfer Pe´clet number (Pe) is observed, which is superior to the logarithmic dependence of mixing length on Pe in chaotic mixers. Having a simple planar structure, the PLM can be easily integrated into lab-on-a-chip devices where passive mixing is needed.

Effect of Surface Conditions on the Adhesion of Crystallization Fouling

In this study, effects of surface conditions in terms of surface roughness and oxide layer, on adhesion of crystallization fouling on heat transfer surfaces were investigated. The experimental results showed that the surface roughness has no obvious effect on the adhesion of crystallization fouling. The polished sample did not present better anti-fouling properties compared to other rough samples. While the formation of Fe2O3 layer on the surface is proved to be able to accelerate the adhesion of calcite fouling with hexagonal structure, because there are similar crystalline structure and lattice parameter between the Fe2O3 and calcite fouling. Therefore, in order to improve the anti-fouling property of heat transfer surfaces, inhibiting the formation of oxide layer is more important than efforts to improve surface roughness.

Optimization and Development for Fin-and-Tube Heat Exchangers With Vortex Generators

In this paper, heat transfer enhancement and pressure loss penalty for fin-and-tube heat exchangers with rectangular winglet pairs (RWPs) were numerically investigated in a relatively low Reynolds number flow. The purpose of this study was to explore the fundamental mechanism between the local flow structure and the heat transfer augmentation. The RWPs were placed with a special orientation for the purpose of enhancement of heat transfer. The numerical study involved three-dimensional flow and conjugate heat transfer in the computational domain, which was set up to model the entire flow channel in the air flow direction. The effects of attack-angle of RWPs, row-number of RWPs and placement of RWPs on the heat transfer characteristics and flow structure were examined in detail. It was observed that the longitudinal vortices caused by RWPs and the impingement of RWPs-directed flow on the downstream tube were important reasons of heat transfer enhancement for fin-and-tube heat exchangers with RWPs. It was interesting to find that the pressure loss penalty of the fin-and-tube heat exchangers with RWPs could be reduced by altering the placement of the same number of RWPs from inline array to staggered array and simultaneously maintain the heat transfer enhancement level. The results showed that the rectangular winglet pairs (RWPs) can significantly improve the heat transfer performance of the fin-and-tube heat exchangers with a moderate pressure loss penalty.

Analysis of Fouling Characteristic in Enhanced Tubes Using Multiple Analogies

This paper provides a comprehensive analysis on cooling tower fouling data taken from seven 15.54 mm I.D. helically ribbed, copper tubes and a plain tube at Re = 16000. A new mathematical model has been developed. The mass transfer coefficient Km is calculated through three analogies, which are Prandtl analogy, Von-Karman analogy, and j factor analogy. Fouling deposition is assumed to be determined by two processes, which are corresponding to heat flux and fluid friction. Von-Karman analogy is proved the best analogy among the three. Series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes were developed. They were applicable to different internally ribbed geometries. The correlations can be directly used to assess the fouling potential of enhanced tubes in actual cooling tower water situations.

Non-Condensable Gases Effect on Heat Transfer Processes Between Condensing Steam and Boiling Water in Heat Exchanger With Multirow Horizontal Tube Bundle

The experimental investigations of non-condensable gases effect on the steam condensation inside multirow horizontal tube bundle of heat exchanger under heat transfer to boiling water were carried out at the large-scale test facility in the Institute for Physics and Power Engineering (IPPE). The experiments were carried out for natural circulation conditions in primary and secondary circuits of the facility at primary circuit steam pressure of Ps1 = 0.34 MPa. The experimental heat exchanger’s tube bundle consists of 248 horizontal coiled tubes arranged in 62 rows. Each row consists of 4 stainless steel tubes of 16 mm in outer diameter, 1.5 mm in wall thickness and of 10.2 m in length. The experimental heat exchanger was equipped with more than 100 thermocouples enabling the temperatures of primary and secondary facility circuits to be controlled in both tube bundle and in the inter-tubular space. The non-condensable gases with different density — nitrogen and helium were used in the experiments. The volumetric content of gases in tube bundle amounted to ε = 0.49. The empirical correlation for the prediction of the relative heat transfer coefficient k/k0 = f (ε) for steam condensation in steam-gas mixture was obtained.