Selected Papers From the Tenth UK National Heat Transfer Conference

The 10th UK National Heat Transfer Conference was held in Edinburgh, Scotland, during 10–11 September 2007 under the auspices of the UK Heat Transfer Committee and was co-organized by the University of Edinburgh and Heriot-Watt University. The conference was part of a series of conferences which were initially organized every 4 years but more recently every 2 years in a synchronized manner with the European Thermal Sciences Conference and International Heat Transfer Conference. Thus, the UK heat transfer community, along with the international participants, can get together every year in a national or European or international heat transfer conference.     

Advancing and receding contact lines on patterned structured surfaces

An experimental investigation of advancing and receding contact lines on patterned surfaces was performed in a controlled environment. Hydrophobic polymers were used to create patterned surfaces to mimic defects and the working fluid was water. Surfaces were prepared with holes or pillars every 200 μm and depth/height from 1 to 11 μm. An optical technique was used to measure contact angle. On smooth (control) surfaces, an advancing or receding contact angle was observed. On the patterned surfaces, pinning and depinning at the defects (holes or pillars, respectively) was observed, with advancing or receding contact lines occurring between these depinning/pinning events. The observed pinning/depinning phenomenon of the contact line was investigated to demonstrate the dynamics of the contact line motion over rough surfaces for a small range of contact line velocity. The competition between the Young unbalanced force and the anchoring forces of the defects is thought to dominate the pinning/depinning process. Stick–slip behaviour of the contact line is observed for larger structures and the results show a strong pinning of the contact line on surfaces with larger defects. The datum contact angle and its deviation were measured and a new concept of scaled energy barrier was calculated for advancing contact lines. This was strongly dependent on defect size. An estimation of the unbalanced Young force per unit length was also made for comparison, which also depended on defect size. This new approach allows new insights into this wetting phenomenon.     

Steps towards the development of an experimentally verified simulation of pool nucleate boiling on a silicon wafer with artificial sites

Nucleate boiling is a very effective heat transfer cooling process, used in numerous industrial applications. Despite intensive research over decades, a reliable model of nucleate pool boiling is still not available. This paper presents a numerical and experimental investigation of nucleate boiling from artificial nucleation sites.The numerical investigation described in the first section of the paper is carried out by a hybrid mechanistic numerical code first developed at the University of Ljubljana to simulate the temperature field in a heated stainless steel plate with a large number of nucleation sites during pool boiling of water at atmospheric pressure. It is now being redeveloped to interpret experiments on pool boiling at artificial sites on a silicon plate and as a design tool to investigate different arrangements of sites to achieve high heat fluxes. The code combines full simulation of the temperature field in the solid wall with simplified models or correlations for processes in the liquid-vapour region. The current capabilities and limitations of the code are reviewed and improvements are discussed. Examples are given of the removal of computational constraints on the activation of sites in close proximity and improvements to the bubble growth model. Preliminary simulations are presented to compare the wall conditions to be used in the experiments on silicon at Edinburgh University with the conditions in current experiments on thin metal foils at Ljubljana.An experimental rig for boiling experiments with artificial cavities on a 0.38 mm thick silicon wafer immersed in FC-72, developed at Edinburgh University, is described in the second part of the paper.     

Gravitational effects on evaporative convection at microscale

Thermocapillary convection induced by phase change (evaporation) has been investigated in confined environment. This paper introduces some insight into the physics of evaporatively-driven thermocapillary convection and emphasizes on the interaction between the observed convection and gravity. Non-equilibrium interfacial conditions lead to temperature/surface tension gradients which drive convective patterns. The latent heat of evaporation leads to an important cooling effect near the triple contact line. Evaporation of volatile liquids in capillary tubes is experimentally investigated to demonstrate the above effects. The size of the capillaries is found to be an important factor in the effect that gravity could have on thermocapillary convection. The oscillatory behaviour observed when buoyancy affects thermocapillary convection could be explained through the coupling between interfacial temperature and the flow within the liquid. The three dimensional nature of the flow structure is found to extend the effect of gravity to the horizontal section of the flow.     

Hydrodynamics and heat transfer during flow boiling instabilities in a single microchannel

Boiling in microchannels is widely considered as one of the front runners in process intensification heat removal. Flow boiling heat transfer in microchannel geometry and the associated flow instabilities are not well understood, further research is necessary into the flow instabilities adverse effect on heat transfer.Boiling is induced in microchannel geometry (hydraulic diameter 727 μm) to investigate several flow instabilities. A transparent, metallic, conductive deposit has been developed on the exterior of rectangular microchannels, allowing simultaneous heating and visualisation.Presented in this paper is data for a particular case with a uniform heat flux of 4.26 kW/m² applied to the microchannel and inlet liquid mass flowrate, held constant at 1.13 × 10^?5 kg/s. In conjunction with obtaining high-speed images, a sensitive infrared camera is used to record the temperature profiles on the exterior wall of the microchannel, and a data acquisition system is used to record the pressure fluctuations over time. Various phenomena are apparent during the flow instabilities; these can be characterised into timescales occurring at 100’s seconds, 10’s seconds, several seconds and finally milliseconds. Correlation of pressure oscillations with temperature fluctuations as a function of the heat flux applied to the microchannel is possible.From analysis of our results, images and video sequences with the corresponding physical data obtained, it is possible to follow simultaneously particular flow, pressure and temperature conditions leading to nucleate boiling, flow instabilities and transition regimes during flow boiling in a microchannel. The investigation allowed us to quantify and characterise the timescales of various observed instabilities during flow boiling in a microchannel. High speed imaging revealed some of the controlling physical mechanisms responsible for the observed instabilities.     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

Unsteady-state fluctuations analysis during bubble growth in a “rectangular” microchannel

Boiling in microchannels shows great potential for cooling systems and compact heat removal applications. However for confidence in this cooling technique, it is essential that any excursions from typical flow boiling are understood and predicted. Confined bubble growth can cause pressure fluctuations which interfere with bubble nucleation and growth and can also lead to flow reversal and instances of temperature excursions. Boiling experiments are performed in a single rectangular microchannel of hydraulic diameter 771 μm, using n-Pentane as the working fluid. A heating technique was incorporated on the exterior walls of the microchannel; a transparent, metallic, conductive deposit, which allows simultaneous uniform heating and visualisation to be achieved. In conjunction with obtaining high-speed imaging, an infrared camera is used to record the temperature profile at the microchannel wall, and sensitive pressure sensors are used to record the pressure drop across the microchannel over time. During flow boiling in the microchannel periodic and non-periodic fluctuations in both the channel pressure drop and channel temperature profile over time are apparent. In this paper we provide a full analysis of the temperature measurements and pressure data obtained during the growth of a vapour bubble in the microchannel. An augmentation of the heat transfer coefficient of over 216% has been achieved during periodic two-phase flow boiling in the microchannel. However overpressure (over 410% increase) in the microchannel occurs at corresponding instances to the heat transfer enhancement. The two time steps during the periodic bubble dynamics, namely the bubble expansion time period and the waiting time period in-between the bubble expansion fluctuations, are also investigated and modelled. It was determined that both the bubble dynamics and the channel wall heating time period are responsible for the pressure and temperature fluctuation time periods observed.     

Investigation of Flow Distribution in Microchannels Heat Sinks

The thermal efficiency of microchannel-based heat sinks relies on uniform fluid flow distribution between channels. Maldistribution, whether caused by poor manifold design or blockage of individual microchannels, can lead to hotspots and consequent thermal damage. This work considers design of manifolds for even flow distribution and the effect of channel blockage on the flow. An approximate model was used to evaluate the effect of manifold geometry on the flow distribution, and the results were compared with computational fluid dynamics (CFD) simulation. Various parameters, which influence the flow distribution such as the shape of distributing and collecting manifolds and position of inlet and outlet holes, have been studied for different inlet flow rates. The effect of channel blockage on flow distribution and pressure drop has been investigated. It was found that good agreement between results of the approximate model and results of CFD simulations are shown only for low Reynolds numbers. Results obtained by approximate model and CFD simulations were used to assist design of manifolds for uniform flow distribution between microchannels.