27Th HEXAG MEETING

27th HEXAG MEETING

HERIOT-WATT UNIVERSITY, EDINBURGH, 2 APRIL 2008.

MEETING MINUTES

The 27th HEXAG meeting was held at Heriot-Watt University on 2nd April 2008, in Mechanical Engineering.

Technical Presentations

Synthetic Air Jets: The technical presentations were kicked off by Tadhg O’Donovan of Heriot-Watt (T.S.O’[email protected]) who described the development of synthetic air jet technology as a means of cooling electronics. Tadhg described the concept, based upon the use of a flexible membrane that is oscillated either mechanically or via Piezo-electric effects. The vortices produced enhance heat transfer on the surface, the synthetic air jet consisting entirely of vortex chains.

Forced air convection for cooling currently handles 0.5-1.9 W/cm2 and is predicted to increase to 10-40 W/cm2 over the next decade. CPU fans (current technology) can give heat transfer coefficients of up to 60 W/m2K and synthetic air jets could give to 96 W/m2K. Tadhg described the characterisation of the jets – the ‘stroke length’, Reynolds number and the Strouhal number. The apparatus used in the laboratory for generating the jet comprised an acoustic speaker diaphragm, and Re was 5300. Particle image velocimetry (PIV) was used to analyse the heat transfer performance.

The next step was to look at the fluctuating heat transfer coefficient (HTC) using a hot film sensor set at 5oC above the plate temperature. The HTC was a function of the distance of the nozzle from the plate and the impact of the vortices. In practice one would need a cluster/array of jets in confined conditions. In order to get the jets to interact constructively the vortices should not cancel each other out, so they are pulsed out of phase.

Preliminary heat transfer results with 2 jets in phase showed some heat transfer occurring, but pulsing out of phase (120o) showed much increased heat transfer over a greater area of surface. The system could out-perform standard fin-fan CPU coolers and one can also operate clusters of jets.

Heat Transfer Challenges in Process Intensification: David Reay ([email protected]) introduced some of the heat transfer challenges that arise as one intensifies unit operations and hence heat and mass transfer. After defining what process intensification (PI) is, he outlined the aspects of it that were relevant to HEXAG members – these include micro-heart exchangers, heat exchanger-reactors, heat pipes and the many methods of enhancement, including rotation and electric fields. Historically, 100-year old rotating refrigerators (high-g forces) and the NASA rotating boiler in the 1960s were illustrated. A device that incorporates rotation to enhance heat and mass transfer, including absorption and evaporation, is the Rotartica chiller, designed originally by Prof. Colin Ramshaw, the inventor of the rotating distillation column at ICI – HiGee. heat exchanger-reactors and highly compact reformers have challenges in terms of heat flux capability and control of uniform temperature distributions. Such reformers are now forming the basis of fuel cells in equipment such as mobile phones. Returning to rotation, David said that spinning disc reactors (SDRs) had to handle rapid highly exothermic reactions on one side with the disc rotating at up to 10,000 rpm. The strong mixing in the thin films associated with the SDR may have benefits in some multi-phase heat transfer applications, as well as creating thermal control challenges – examples where rapid mixing has been a process benefit include custard making!

After discussing the heat transfer on the back of the SDR, and showing micro-heat exchangers in ceramics, David introduced a cross-flow metal mesh heat exchanger developed at Cambridge University. Superior performance (Nu~Re) was exhibited compared to other surface configurations for both water and air as fluids, particularly with copper woven mesh structures. Heat pipes also can help to overcome some of the heat transfer limitations in conventional chemical/bio-reactors by distributing heat within the reactor bed in order to even out reaction rates and hence product quality. Electronics thermal control is also the target of intensification (as discussed by Tadhg above) and both active and passive intensification methods, including micro-vapour compression cycles on the back of chips and loop heat pipes, are used to help handle the increasing heat dissipation needs.

Compact Heat Exchangers in Evaporator Systems: Fred Brotherton of BEEDES Ltd ([email protected]) described the use of compact heat exchangers in the food & drink sector to reduce energy use. The energy use in the food & drink sector is about 15% of that in the process industries in general, and within the sector more than 60% of the energy used is for the three unit operations of evaporation, drying and distillation. A large number of these processes take place at less than 100oC.

Fred said that plate heat exchangers (PHEs) can now tolerate high pressures, gasketed units tolerating 20-25 bar with 0.5 – 1.0 mm thick plates. He described how plates were supported and clamped. Generally counter-current flow was employed, and one can have many asses to achieve the required hat transfer. While in some designs the contact points between plates can attract suspended solids in the heat exchanger leading to blockage, new ‘freeflow’ plates overcome this. In this design the flow channels are free from contact points.

A specific application where PHEs could be used to benefit energy efficiency is in effluent evaporation, the use described being for pot ale concentration where the recovered effluent has a high value as animal feed. In the original plant design the hot water flows were complex and there as high product recirculation. The system operated under vacuum. One improvement was to take out the flash stages completely and replace shell & tube heat exchangers with PHEs, and nothing needed to operate under vacuum. The outcome was less pumping power, a higher condenser pressure, higher efficiency and easier operation.

By combining the PHEs and shell & tube heat exchanger with the PHEs being used with hot water on one side and as a falling film evaporator on the other, a hybrid system with substantial benefits would result. The outcome was a three-effect evaporator incorporating three PHEs that helped to optimise the hot water and product temperature profiles.

2 Fred said that the next stage at this plant was to examine what could be done with the exhaust from the dryer. There is considerable latent heat potential, but at a low temperature. One could design an evaporator to be heated by the dryer exhaust, but a large non-condensible gas flow would be present and conventional evaporators are not designed to accommodate this. An alternative might be to scrub the exhaust to recover hot water.

The Microstructure of Solid Surfaces and Effect on Two-phase Flow: Andrea Luke from the Institut fur Thermodynamik in Hannover ([email protected]) described her work on investigating bubbles and bubble formation in evaporation, particularly on tube surfaces, with a view to intensifying heat transfer. She described the advantages and disadvantages of evaporation and the various boiling mechanisms – pointing out that each of them was influenced by the surface micro-structure. Ideally one needs to shift the boiling characteristic curve towards lower superheats, and many parameters can influence this – the fluid properties, the surface type and the material, etc.

Andrea described the nature of surfaces, including those with conical cavities or re-entrant cavities which form potential sites for nucleation. Manufactured surfaces such as roughened tubes are also an option. Studies of the distribution and size of cavities for the best nucleation are being undertaken, and ‘cavity maps’ are being drawn. Two examples were from emery- ground surfaces (which had a broad size distribution) and fine sand-blasted surfaces that exhibited a better narrow size distribution. The highest heat transfer coefficient was achieved with emery, but this was achieved at a high heat flux. High speed videos showed the bubble formation and movement in these cases, the heat flux being 20 kW/m2. An automatic image analysis system is being used in an attempt to count the bubbles and Andrea had looked at the variations in site activation as a function of time.

The aim of the research is to develop a universally valid theory of heat transfer in boiling by the use of models of the transport process. Research is being carried out on GEWA high flux tubes.

Solar PV Concentrators with Passive Cooling: Deborah Adkins of Nottingham University ([email protected]) introduced us to her research on cooling of photo-voltaic (pv) solar concentrators. She said that cooling was important to allow optimisation of current designs and to help generate new collector configurations. The main area of work is on concentrators, where the pv semiconductor cost becomes less than 20% of the total collector cost – allowing high performance pv cells to be used. They would be too expensive for flat plate pv systems. Concentrators may be refractive or reflective.

Deborah showed a graph showing the efficiencies of a wide range of solar pv systems. These could reach over 40% for a concentrator using multi-junction cells, and much lower efficiencies were achieved with cheap organic pv cells. Deborah is looking at the mid- concentration range, which requires higher tracking tolerances but can use lower cost optics.

Thermal control of pv systems is important from both a solely thermal viewpoint and a thermo-mechanical one. The band gap of a semiconductor shrinks as temperature increases, leading to a reduction in efficiency due to a reduced current density. Decisions also relate as to whether to employ passive or active cooling systems. The need for cooling also allows one to look at a combined pv/thermal system.

3 Experiments were carried out on the thermal control system both indoors (using a solar simulator) and outdoors. Deborah showed the trough concentrator, with a finned heat sink on the back used for heat removal. Two-and three-dimensional modelling was carried out using software such as Fluent etc.

Flow Maldistribution in Plate Heat Exchangers: The final talk at the meeting returned to the topic of PHEs. Prabhakara Roa Bobbili of Heriot-Watt University ([email protected]) first explained why it was important to study this topic and highlighted the areas causing maldistribution, dealing in particular with single phase duties. Maldistribution may be geometry-induced, affecting gross flow or passage-to-passage flows; it can be manifold- induced or the operating conditions may induce flow maldistribution.

The main parameters of PHEs are channel size, flow rates and the diameter of the port. Rao said that the latter was important. He showed experimental data on an eight channel PHE which exhibited non-uniform distribution, and a 21 channel unit that gave uniform distribution. For a 32 channel unit Rao demonstrated the impact that maldistribution could have on the heat exchanger effectiveness.

Following the talks, there was a tour of laboratories involved in work flow boiling, multi-tube condenser research, micro-boiling research, acoustic refrigeration, synthetic jet cooling (in Mechanical Engineering) and micro-cooling plates (in the Microsystems Engineering Centre).

Most of the overheads from the talks will be added to the HEXAG web site – www.hexag.org in due course.

NEXT HEXAG MEETING The 28th HEXAG meeting will be held at Brunel University, Uxbridge on 14 October 2008. Offers of talks are invited. Please email David Reay on [email protected] with suggested titles.

Minutes prepared by David Reay, 12 May 2008.