A general method for calculation of flow boiling and flow condensation heat transfer coefficients in minichannels

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.Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena, however their description with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where bubble generation is not present. Similar picture holds for inside tube condensation, where annular flow structure predominates. In such case the heat transfer coefficient is primarily dependent on the convective mechanism. In the paper a method developed earlier by D. Mikielewicz et al. (2007) is applied to calculations of heat transfer coefficient for inside tube condensation. Satisfactory consistency with well established correlations for condensation has been found

Comparative study of heat transfer and pressure drop during flow boiling and flow condensation in minichannels

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.In the paper a method developed earlier by authors is applied to calculations of pressure drop and heat transfer coefficient for flow boiling and also flow condensation for some recent data collected from literature for such fluids as R245fa, R600a, R134a, R1234yf and other. The modification of interface shear stresses between flow boiling and flow condensation in annular flow structure is considered through incorporation of the so called blowing parameter. The shear stress between vapor phase and liquid phase is generally a function of non-isothermal effects. The mechanism of modification of shear stresses at the vapor-liquid interface has been presented in detail. In case of annular flow it contributes to thickening and thinning of the liquid film, which corresponds to condensation and boiling respectively. There is also a different influence of heat flux on the modification of shear stress in the bubbly flow structure, where it affects the bubble nucleation. In that case the effect of applied heat flux is considered. As a result a modified form of the two-phase flow multiplier is obtained, in which the non-adiabatic effect is clearly pronounced

Management of low grade waste heat from the supercritical power plant using the ORC installation aided by bleed steam

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.In the paper presented is a novel concept to utilize the heat from the turbine bleed to improve the quality of working fluid vapour in the bottoming ORC installation. That is a completely novel solution in the literature, which contributes to the increase of ORC efficiency and the overall efficiency of the combined system of the power plant and ORC plant. Calculations have been accomplished for the case when available is a flow rate of low enthalpy hot water at a temperature of 90°C, which is used for preliminary heating of the working fluid. That hot water is obtained as a result of conversion of exhaust gases in the power plant to the energy of hot water. Then the working fluid is further heated by the bleed steam to reach 120°C. Such vapour is subsequently directed to the turbine. In the paper 5 possible working fluids were examined, namely R134a, MM, MDM, toluene and ethanol. In all cases the ethanol proved to be best performing fluid of all. Results are compared with the “stand alone” ORC module showing its superioritydc201

Influence of metallic porous microlayer on pressure drop and heat transfer of stainless steel plate heat exchanger

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 experimental analysis of passive heat transfer intensification in the case of plate heat exchanger has been carried out. On the heat transfer surface of heat exchanger the metallic porous layer was created. The experiment was accomplished in two stages. In the first stage the commercial stainless steel gasketed plate heat exchanger was investigated, while in the second one – the identical heat exchanger but with the modified heat transfer surface. The direct comparison of thermal and flow characteristics between both devices was possible due to the assurance of equivalent conditions during the experiment. Equivalent conditions mean the same volumetric flow rates and the same media temperatures at the inlet of heat exchangers in the corresponding measurement series. Experimental data were collected for the single-phase convective heat transfer in the water-ethanol configuration. The heat transfer coefficients were determined using the Wilson method

A microjet based recuperator for application in domestic micro chp

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.In the paper the original design of a compact heat exchanger with microjets producing intensification effect is presented. Its primary application is for the domestic Organic Rankine Cycle (ORC), however, the design is universal and may have numerous other applications. The technology of microjets manufacturing is an “in-house” patented design. In the present paper the idea of such a heat exchanger is shown together with the flow and thermal characteristics of the prototype. The developed prototype of heat exchanger is capable of exchanging 5 kW of thermal energy at a logarithmic mean temperature difference (LMTD) of 60 K. The total heat transfer surface equal to 0.0072 m2 leads to very significant heat fluxes. Measured overall heat transfer coefficient reaches 12000 W/m2K, which was calculated using the Wilson method. The description of the Wilson technique used for the determination of the heat transfer coefficient is also presented in the body of the text. That method seems to be, in the authors’ opinion, the only one for finding the heat transfer coefficient for such a complex heat exchanger structure. In this case measurements of wall temperatures are not possible and hence the determination of heat transfer coefficient is difficult. The results of performed measurements are satisfactory and encourage for further research of the original design.pm201

Effect of surface finish on heat transfer performance of plate heat exchanger

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.In the paper an experimental analysis of passive heat transfer intensification technique employed in the case of plate heat exchanger is presented. The passive intensification was obtained by a modification of the heat transfer surface. The roughness of surface was increased by a usage of glass micro- beads. Single-phase convective heat transfer in the water-water system was studied. The experiment was accomplished in two stages. In the first stage the commercial plate heat exchanger was investigated, while in the second one – the identical heat exchanger but with the modified heat transfer surface. The direct comparison of thermal and flow characteristics between both devices was possible due to the assurance of equivalent conditions during the experiment. Equivalent conditions mean the same volumetric flow rates and the same media’s temperatures at the inlet of heat exchangers in the corresponding measurements’ series. Due to this the systematic experimental data show that larger roughness of heat transfer surface leads to an increase of heat transfer coefficient on the side of cooling water (increase by about 30 ÷ 35%) and simultaneously to an increase of flow resistance (up to 30% when the volumetric flow rate is equal to 500 l/h). On the side of heating water it was found that the heat transfer coefficient increased by about 25%, while the flow resistance by about 22% (the volumetric flow rate of 500 l/h).dc201

The energetics of the breakup of a sheet and of a rivulet on a vertical substrate in the presence of a uniform surface shear stress

We use the lubrication approximation to obtain a complete description of the energetics of the breakup both of a fluid sheet of uniform thickness into a periodic array of infinitely many identical thin rivulets and of a single thin rivulet into one or more identical sub-rivulets on a vertical substrate in the presence of a prescribed uniform longitudinal shear stress at the free surface of the fluid by comparing the total energies of the different states. For both problems the situation when the volume flux is positive is relatively straightforward (and, in particular, qualitatively the same as that in the case of no prescribed shear stress), but when the volume flux is negative it is more complicated. However, whatever the value of the prescribed shear stress, there is always a critical thickness below which it is energetically favourable for a sheet to break up into rivulets and a critical semi-width below which it is energetically favourable for a rivulet to remain as a single rivulet, and a critical thickness above which it is energetically favourable for a sheet to remain as a sheet and a critical semi-width above which it is energetically favourable for a rivulet to break up into sub-rivulets

Heat transfer enhancement, flow visualization and friction characteristics in rib-roughened channels

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.In the last two decades thermochromic liquid crystal (TLS), true-color digital image processing and laser anemometry (PIV) have been successfully used in non-intrusive, technical and heat and mass transfer studies and applications. The aim of the paper is to asses the state of the art in the exploration of heat transfer control by transverse vortex generators. They are able to increase heat transfer by several hundred percent. Prior to the use of vortices to influence heat transfer it must be known how different vortices are generated and controlled and how they interact with the original or base flow and temperature field. Liquid crystals were used to determinate the distribution of surface temperature and then evaluation of heat transfer coefficient or the Nusselt number. The flow pattern produced by transverse vortex generators (rib-roughened passages) was visualized using a planar beam of double-impulse laser tailored by a cylindrical lens and oil particles. Sequential images of particles in the cross sectional plane taken with CCD video camera from the downstream side the flow were stored on a personal computer to obtain distributions of velocity vectors by means of the PIV method. Local and average Nusselt numbers, friction factors and velocity fields are presented for rectangular channels with an aspect ratio of 6.35 and five types of transverse vortex generators which were brought to required temperatures by hot film method. The pitch-to-height-ratio of the ribs was 11.mp201

White light thermoplasmonic activated gold nanorod arrays enable the photo-thermal disinfection of medical tools from bacterial contamination

The outspread of bacterial pathogens causing severe infections and spreading rapidly, especially among hospitalized patients, is worrying and represents a global public health issue. Current disinfection techniques are becoming insufficient to counteract the spread of these pathogens because they carry multiple antibiotic-resistance genes. For this reason, a constant need exists for new technological solutions that rely on physical methods rather than chemicals. Nanotechnology support provides novel and unexplored opportunities to boost groundbreaking, next-gen solutions. With the help of plasmonic-assisted nanomaterials, we present and discuss our findings in innovative bacterial disinfection techniques. Gold nanorods (AuNRs) immobilized on rigid substrates are utilized as efficient white light-to-heat transducers (thermoplasmonic effect) for photo-thermal (PT) disinfection. The resulting AuNRs array shows a high sensitivity change in refractive index and an extraordinary capability in converting white light to heat, producing a temperature change greater than 50 °C in a few minute interval illumination time. Results were validated using a theoretical approach based on a diffusive heat transfer model. Experiments performed with a strain of Escherichia coli as a model microorganism confirm the excellent capability of the AuNRs array to reduce the bacteria viability upon white light illumination. Conversely, the E. coli cells remain viable without white light illumination, which also confirms the lack of intrinsic toxicity of the AuNRs array. The PT transduction capability of the AuNRs array is utilized to produce white light heating of medical tools used during surgical treatments, generating a temperature increase that can be controlled and is suitable for disinfection. Our findings are pioneering a new opportunity for healthcare facilities since the reported methodology allows non-hazardous disinfection of medical devices by simply employing a conventional white light lamp

Influence of Pachymetry and Intraocular Pressure on Dynamic Corneal Response Parameters in Healthy Patients

To evaluate the influence of pachymetry, age, and intraocular pressure in normal patients and to provide normative values for all dynamic corneal response parameters (DCRs) provided by dynamic Scheimpflug analysis