Evaporation of a thin droplet on a thin substrate with a high thermal resistance

A mathematical model for the quasi-steady evaporation of a thin liquid droplet on a thin substrate that incorporates the dependence of the saturation concentration of vapour at the free surface of the droplet on temperature is used to examine an atypical situation in which the substrate has a high thermal resistance relative to the droplet (i.e. it is highly insulating and/or is thick relative to the droplet). In this situation diffusion of heat through the substrate is the rate-limiting evaporative process and at leading order the local mass flux is spatially uniform, the total evaporation rate is proportional to the surface area of the droplet, and the droplet is uniformly cooled. In particular, the qualitative differences between the predictions of the present model in this situation and those of the widely used 'basic' model in which the saturation concentration is independent of temperature are highlighted

A study of nucleate boiling and critical heat flux with EHD enhancement

The paper describes results from an experimental and theoretical study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling of R123 at atmospheric pressure on a horizontal wall with a smooth surface. Two designs of electrode (parallel rods and wire mesh) were used. The experimental data exhibit some differences from the data obtained by other researchers in similar experiments on a wall with a different surface finish and with a slightly different design of wire mesh electrode. The hydrodynamic model for EHD enhancement of CHF cannot reconcile the differences. A theoretical model has been developed for the growth of a single vapour bubble on a superheated wall in an electric field, leading to a numerical simulation based on the level-set method. The model includes matching of sub-models for the micro- and macro- regions, conduction in the wall, distortion of the electric field by the bubble, the temperature dependence of electrical properties and free-charge generation. In the present form of the model, some of these effects are realised in an approximate form. The capability to investigate dry-spot formation and wall temperature changes that might lead to CHF has been demonstrated

Modelling of the growth and detachment of a vapour bubble and the effect of a electric field in the nucleate boiling regime

A comprehensive model predicting the deformation, growth and detachment of a vapour bubble in the nucleate boiling regime with an applied electric field is described in this paper. The model takes into account the full electrohydrodynamics of the phenomenon including the influence of local temperature on the generation of free charges in the liquid. Solution of the model by the level set method has been successfully implemented with a commercial CFD code. Aspects of the code and the graphical software requiring further development are noted. Sample results are presented to demonstrate the effect of the electric field on the growth and detachment of the bubble, for a bubble initially protruding through a thermal boundary layer on a horizontal wall. The bubble is elongated under the influence of electrical forces, the effect being more pronounced for stronger electrical fields. The electric field is found to promote earlier detachment of the bubble at a smaller volume, thus increasing the bubble frequency. The wall heat flux during the process of detachment is not much affected by the electric field

On the effect of the atmosphere on the evaporation of sessile droplets of water

An experimental and theoretical study into the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described. The experimental work investigated the evaporation rates of sessile droplets in atmospheres of three different ambient gases (namely, helium, nitrogen and carbon dioxide) at reduced pressure (from 40 to 1000 mbar) using four different substrates(namely, aluminium, titanium, Macor and PTFE) with a wide range of thermal conductivities.Reducing the atmospheric pressure increases the diffusion coefficient of water vapour in the atmosphere and hence increases the evaporation rate. Changing the ambient gas also alters the diffusion coefficient and hence also affects the evaporation rate. A mathematical model that takes into account the effect of the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapour in the atmosphere and the thermal conductivity of the substrate is developed, and its predictions are found to be in encouraging agreement with the experimental results

Nucleate pool boiling investigation on a silicon test section with micro-fabricated cavities

The basic mechanisms of nucleate boiling are still not completely understood, in spite of the many numerical and experimental studies dedicated to the topic. The use of a hybrid code allows reasonable computational times for simulations of a solid plate with a large population of artificial micro-cavities with fixed distribution. This paper analyses the guidelines for the design, through numerical simulations, of the location and sizes of micro-fabricated cavities on a new silicon test section immersed in FC-72 at the saturation temperature for different pressures with an imposed heat flux applied at the back of the plate. Particular focus is on variations of wall temperature around nucleation sites

Polydimethylsiloxane (PDMS)-based microfluidic channel with integrated commercial pressure sensors

The precise characterisation of boiling in microchannels is essential for the optimisation of applications requiring two phase cooling. In this paper polydimethylsiloxane (PDMS) is employed to make microchannels for characterising microboiling. In particular the material properties of PDMS facilitate rapid prototyping and its optical transparency provides the capability to directly view any fluid flow. The production of microchannels is complicated by the need to integrate custom made sensors. This paper presents a PDMS microfluidic device with integrated commercial pressure sensors, which have been used to perform a detailed characterisation of microboiling phenomena. The proposed approach of integrating commercial pressure sensors into the channel also has potential applications in a range of other microsystems

Velocity measurement during evaporation of seeded, sessile drops on heated surfaces

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Evaporation of sessile drops has been studied extensively in an attempt to understand the effect of wetting on the evaporation process. Recently interest has also increased in the deposition of particles from such drops, with evaporative mass flux being deemed to be responsible for ring-like deposits and Marangoni convection counteracting this mass flux explaining more uniform deposition patterns. Understanding of such deposition processes is important in ink-jet printing and other micro-scale deposition technologies, where the nature of deposition can have a dramatic effect on the quality or effectiveness of the finished product. In most cases where deposition from evaporating drops has been studied, velocity information is inferred from the final deposition pattern or from mathematical modeling based on simplified models of the physics of the evaporation process. In this study we have directly measured the flow velocities in the base of sessile drops, using micro-PIV, viewing the drop from below, through the cover slide. The images obtained have also enabled us to observe the formation of holes in the liquid film during the latter stages of evaporation

The leidenfrost phenomenon on structured surfaces

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The lifetime of a droplet released on a hot plate decreases when the temperature of the plate increases. But above some critical value of the temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapour between the droplet and the substrate. This layer plays a double role: first it thermally isolates the droplet from the plate and second it allows the droplet to “levitate.” This effect was discovered by Leidenfrost in 1756, but remains an active field of research nowadays, motivated by a wide range of applications. The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different micro-structured regions of square-pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water drops ranging in average size from 8 μL to 24 μL and the behaviour of the droplets was recorded using a high-speed digital camera. We show that the Leidenfrost point can vary by up to 120 °C for pillar spacings varying from 10 microns to 100 microns. If the drop is placed on the boundary between structured sections, the drop becomes asymmetric. Drop motion may also be observed and some occurrences of drop spinning have been seen. In this paper we present experimental data on Leidenfrost behaviour of drops placed structured surfaces and on the boundary between surfaces with different micro-structures

Experimental investigation of non-uniform heating on flow boiling instabilities in a microchannels based heat sink

Two-phase flow boiling in microchannels is one of the most promising cooling technologies able to cope with high heat fluxes generated by the next generation of central processor units (CPU). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as a cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated, with hotspots located in different positions along the heat sink. It was observed that boiling inside microchannels with non-uniform heating led to high temperature non-uniformity in transverse direction

Recent progress on the investigation of phase change and interfacial conditions in microsystems

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Evaporation of liquids is a fundamental phenomenon pertaining to a wide range of industrial and biological processes. In this paper we present recent results on evaporation of liquids and interfacial phenomena in the case of two configurations: menisci in micro-channels and sessile wetting droplets . Interfacial temperature is a key factor in the phase change process. The access to the interface temperature at the micro-scale has been a challenging task. Recently Ward and Duan [22] have investigated the cooling effect resulting from the evaporation of water in a reduced pressure environment by using micro-thermocouples near the interface. They show an increase in the cooling effect with the increase in the evaporation mass flux. They also show an important experimental result that is in contrast with classical kinetic theory and non-equilibrium thermodynamics. The temperature in the vapour phase is higher than in the liquid phase. The authors also discussed the fundamental question about the interfacial conditions during phase change. Indeed, as instrumentation has developed, it has become possible to make measurements of the temperature within one-mean-free path of the interface of water as it evaporates steadily, and these measurements have not supported the prediction from classical kinetic theory that the interfacial vapour temperature less than the interfacial liquid temperature. In a first part of the paper, we present data from an experimental study that has been performed to investigate the evaporation of a liquid in a capillary microchannel. The phase change has been found to induce convection patterns in the liquid phase below the meniscus interface. The liquid convective structure has been revealed using m-PIV technique. When extra heating is supplied to the system, the convection pattern is altered and eventually reversed depending on the relative position of the heating element with respect to the liquid-vapour interface. An IR thermography is used to measure temperature gradients generated by the heater along the capillary wall and of the interface. This allowed us to investigate the relation between the temperature gradients applied along the wall and the convection taking place in the liquid under the thermocapillary stress hence generated. In the second part of the paper we investigate the complexity of the evaporative process of wetting drops by means of IR thermography. The obtained data for volatile sessile drops clearly show that there are phenomena at work which, whilst invisible to the naked eye, may have a great importance in many evaporation dependent areas. The naturally occurring thermal instabilities (wave like thermal fluctuations) shown by many investigated working fluids are clearly distinct from each other, and can also be manipulated by altering the evaporation parameters such as substrate material and substrate temperature. What is also interesting to note is that whilst these waves have been observed for these relatively volatile liquids, there appears to be no such behaviour in pure water droplets. The visual observations presented in this paper form the basis for which a full systematic analysis of the wave behaviour can be achieved. Wave number, frequency, velocity, and amplitude are all parameter which can be measured and then used to characterise the behaviour of each fluid. The above described phenomena are entirely self-generated by the phase change process.vk201