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A review of evaporative cooling system concepts for enginethermal management in motor vehiclesArticle (Accepted Version)Jafari, Soheil, Dunne, Julian F, Langari, Mostafa, Yang, Zhiyin, Pirault, Jean-Pierre, Long, Chris Aand Thalackottore Jose, Jisjoe (2017) A review of evaporative cooling system concepts forengine thermal management in motor vehicles. Proceedings of the Institution of MechanicalEngineers, Part D: Journal of Automobile Engineering, 231 (8). pp. 1126-46. ISSN 0954-4070This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/63582/This document is made available in accordance with publisher policies and may differ from thepublished version or from the version of record. If you wish to cite this item you are advised toconsult the publisher’s version. Please see the URL above for details on accessing the publishedversion.Copyright and reuse:Sussex Research Online is a digital repository of the research output of the University.Copyright and all moral rights to the version of the paper presented here belong to the individualauthor(s) and/or other copyright owners. To the extent reasonable and practicable, the materialmade available in SRO has been checked for eligibility before being made available.Copies of full text items generally can be reproduced, displayed or performed and given to thirdparties in any format or medium for personal research or study, educational, or not-for-profitpurposes without prior permission or charge, provided that the authors, title and full bibliographicdetails are credited, a hyperlink and/or URL is given for the original metadata page and thecontent is not changed in any way.http://sro.sussex.ac.uk

1A REVIEW OF EVAPORATIVE COOLING SYSTEM CONCEPTS FOR ENGINETHERMAL MANAGEMENT IN MOTOR VEHICLESbyS. Jafari, J. F. Dunne*, M. Langari, Z. Yang,J-P Pirault, C. A. Long, and J. Thalackottore JoseDepartment of Engineering and DesignSchool of Engineering and InformaticsUniversity of Sussex, Falmer, Brighton, BN1 9QT, UK.* Corresponding author: E-mail: [email protected]

2ABSTRACTEvaporative cooling system concepts proposed over the past century for engine thermalmanagement in automotive applications are examined and critically reviewed. Thepurpose of the review is to establish evident system shortcomings and to identifyremaining research questions that need to be addressed to enable this importanttechnology to be adopted by vehicle manufacturers. Initially, the benefits of evaporativecooling systems are restated in terms of improved engine efficiency, reduced CO2emissions, and improved fuel economy. An historical coverage follows of the proposedconcepts dating back to 1918. Possible evaporative cooling concepts are then classifiedinto four distinct classes and critically reviewed. This culminates in an assessment of theavailable evidence to establish the reasons why no system has yet made it to serialproduction. Then, by systematic examination of the critical areas in evaporative coolingsystems for application to automotive engine cooling, remaining research challenges areidentified.32 main-section pages (double spaced) 72 referencesFigures 1 – 13 No appendices

31.INTRODUCTIONEvaporative Cooling (EC) is an effective means of thermal management and temperaturecontrol in a very diverse range of natural and man-made application areas - from thesmallest of mammals to large electrical-power generation plants. Current engineeringapplications of this technology can be found in a number of important areas, for example:the air-conditioning of buildings in countries with low-humidity climates; in nuclear powerplant reactors; and more recently, for cooling electronic hardware in personal computers.The application of EC to automotive combustion engine thermal management has seen,over the past century, numerous system concepts proposed, patented, analysed,prototyped, and in some cases implemented. Yet none of the proposed systems haveactually made it to serial production. This review will examine the proposed concepts froman historical viewpoint and then put them into appropriately defined classes. It will thenattempt to understand the reasons why (from a technology development viewpoint)existing EC system concepts have all effectively become stuck at a low technologyreadiness level with the result that none of the proposed systems have actually beenimplemented by vehicle manufacturers. The review will then address the remainingresearch challenges associated with EC systems, which if solved, will hopefully removethe major obstacles to implementation.Figure 1 shows a schematic diagram of an automotive EC system. Water, in the liquidphase is introduced into the cooling jacket of the engine and a control system allows thiswater to boil off and become vapour. The liquid and vapour phases are separated in aseparator tank and the vapour is returned to the liquid state in a condenser. A pump isused to circulate the flow around the system and to minimise pumping power, this pumpsthe liquid phase - not vapour.

4The underlying principle behind all engine EC concepts is to exploit the substantial heattransfer rates that occur with the liquid-to-vapour phase-change that results during boiling.This provides a significant enhancement over conventional liquid-based engine coolingsystems which remove heat largely through single phase convective heat transfer(although state-of-the-art ‘subcooled’ systems experience some breakthrough boiling).Engine cooling technologies, including conventional and subcooled systems have, fromdifferent perspectives, been excellently-reviewed in 2004 in [1], and in 2005 in [2], andmore recently in 2010 [3]. Conventional systems are however reaching their limits forefficient thermal management, especially for aggressively-downsized highly-boostedengines. This limitation motivates the search for alternative cost-effective, efficient,durable, and controllable cooling systems. EC systems could overcome this limitation ifthe obstacles and challenges can be identified and overcome because, as discussedbelow, the benefits are clear.It is clear that in an automotive engine EC system, the liquid-to-vapour phase changemust take place within the cylinder jacket. In a typical concept design (and there areseveral distinct designs possible) the vapour formed must be vented, captured, andcondensed from its gaseous state and returned to the cooling circuit as a liquid. Heat fromthe engine is therefore absorbed by exploiting the latent heat of vaporisation of thecoolant. This results in several major advantages:1) a reduction in coolant mass and overall system size;2) lower coolant flow rates (with consequential lower pumping power losses);3) uniform cylinder-head temperatures;4) better knock control;5) reduced noxious emissions;6) reduced parasitic losses (e.g. cooling pump) and lower friction.

5These advantages, in turn lead to improved overall engine efficiency, reduced CO2emissions, and improved fuel economy.Over the past century (i.e. from the earliest evidence of EC system studies forautomotive engine temperature control [4]), the period from 1918 to 1960 can be viewedas a modest period of pioneering work. During this time very few patents were granted,however, towards the end of the period (1958), a major US Navy report by Beck [5],comprehensively reviewed the prospects for EC of internal combustion engines. Theperiod from 1962 to 1990 can be seen as one of more positive growth for automotive ECsystems with five patents granted. During this period, i.e. in 1983, a comprehensive studyof evaporative engine cooling was published by Leshner [6]. Since 1990 the level ofgrowth has continued with nine patents being granted and the number of publishedresearch studies has also increased. For example, the significant analytical andexperimental study published in 1997 by Porot et al [7] aimed to better understand andimprove evaporative engine cooling at high engine loads and speed.Although the fundamental physics of vapour-bubble formation within a saturated fluid isnot yet fully understood [8] (especially the heat flux associated with bubble departure froma hot surface) it is still possible to logically classify EC systems concepts; the classificationbeing based on system components. The main objectives of this review are therefore tohelp put the historical concepts into context by defining distinct EC system classes, andthen to undertake a systematic examination of the critical areas in EC systems to identifythe remaining research challenges for automotive engine cooling applications. To start offthis process, a summary of heat transfer in boiling is given, followed by a brief discussionof simulation and modelling of boiling and two phase flow, culminating in a furthertechnical discussion of the benefits of EC for IC engines.

62.THE PHYSICAL PRINCIPLES,EVAPORATIVE COOLINGCFDMODELLINGANDBENEFITSOF2.1 Heat Transfer in BoilingBoiling takes place at a solid-liquid boundary, distinguishing it from the process ofevaporation which occurs at a liquid-vapour boundary. The physical mechanismsassociated with the vigorous production of vapour bubbles and enhanced heat transferrates take place when the heated surface temperature is higher than the saturationtemperature of the liquid, Tsat. However, boiling also occurs when the bulk liquidtemperature is below the saturation temperature. This is referred to as sub-cooled boiling it is restricted to the thin layer adjacent to the heated surface, and as the vapour bubblesmove through the sub-cooled liquid, they collapse and condense. Boiling without anyexternally-imposed flow or agitation is known as pool boiling. Flow boiling is the namegiven when there is superimposed flow or agitation. The heat transfer processes in an ICengine with evaporative cooling are expected to involve a combination of both pool andflow boiling. There are similarities between these two, thus it is appropriate to first considerthe fundamentals of pool boiling before the more complex phenomena associated withflow boiling.Figure 2 shows, for pool boiling of pure water at 1 bar, the heat flux q as a function ofexcess temperature T Tw – Tsat, with both plotted on logarithmic scales [9]. The gradientof the curve gives the heat transfer coefficient. There are four distinct pool boiling regimeswhich occur as the excess temperature T is increased, namely: i) free (or natural)convection, ii) nucleate boiling, iii) transition boiling, and iv) film boiling.Free or natural convection, takes place with small excess temperature (typically T 4 C), where fluid motion is generated by buoyancy forces. Single-phase heat transfercorrelations for free convection may be used in this regime. As T is increased, individual

7vapour bubbles begin to form at nucleation sites and rise through the body of the liquid.This marks the start of the nucleate boiling regime. A further increase in the excesstemperature causes an increase in the production of these vapour bubbles, acorresponding increase in fluid motion, and a rise in the heat flux. The inflection point at T 10 C is significant because it indicates a change from individual bubble formation tooccurrence of large columns of vapour. The increase in thermal resistance associated withthese larger entities causes a reduction in the rate of increase of the heat transfercoefficient. The heat flux reaches a maximum of qmax 1.2 MW/m2 at T 30 C. Beyondthis, transition boiling occurs where an unstable film of vapour covers the surface. Anincrease in T causes a reduction in the heat flux and a minimum value of heat flux, qmin,occurs at the so-called ‘Leidenfrost Temperature’. Beyond this, film boiling occurs with astable film of vapour covering the surface. For these larger values of T, radiation issignificant and should be taken into account in heat transfer calculations.Providing the relevant physical properties of the liquid and vapour phases are known,namely: dynamic viscosity, latent heat of vapourisation, surface tension, density, specificheat, and thermal conductivity, then the heat transfer rates in nucleate boiling and filmboiling, and the values of the heat fluxes qmax and qmin can be obtained from the followingwell-established correlations i.e.: i) Nucleate boiling: Rohsenow [10]; ii) Film Boiling:Berenson [11]; iii) Maximum heat flux, qmax: Lienhard and Dhir [12] and iv) Minimum heatflux, qmin: Zuber [13]. These are also available in most heat transfer textbooks. In thenucleate boiling regime, the heat flux also depends on the nature of the surface, which ischaracterised by an empirical constant (usually tabulated).For flow boiling, the different regimes of flow and heat transfer are generally delineatedby the vapour quality (or dryness fraction) x. The heat flux in flow boiling is usuallyexpressed as a summation of two contributions: i.e. single-phase forced convection, and

8that due to boiling. For pure liquid (x 0%) entering a heated vertical tube, as the fluidproceeds upwards it will undergo nucleate boiling. Initially this occurs with the formation ofindividual bubbles, then with larger ‘slugs’ of vapour which eventually coalesce towards thecentre of the tube forming an annular flow regime where a vapour core exists, the wallsare coated with low thermal resistance liquid, leading to high heat transfer coefficientvalues which strongly depend on the fluid properties. For larger values of dryness fraction(typically for x 25%) there is a transition to a droplet flow regime. This is also associatedwith a significant reduction in the heat transfer coefficient as a consequence of theincrease in the thermal resistance of the fluid (now vapour) adjacent to the walls. Thisoccurs at the so-called “critical heat flux” and is of obvious significance to enginedesigners. Eventually (for a long enough tube), the vapour quality reaches 100%. Then,single-phase forced convection correlations may be used, based on the properties of thesuperheated vapour. A relatively simple correlation for flow boiling heat transfer in avertical tube is given by Klimenko [14]. Qualitatively, the flow regimes inside a horizontalheated tube are similar to those in a vertical one. However, the interplay between theinfluence of buoyancy and fluid velocity serves to make delineation of the flow regimesmore complex than in a vertical tube. Not surprisingly the physics of the flow behaviourand quantification of the heat transfer is more complex than in pool boiling. HoweverGhiaasiaan (2008) [15] provides an excellent review of flow boiling regimes and usefulheat transfer correlations.2.2 Simulation and Modelling of Boiling and Two Phase FlowThe boiling phenomenon and two-phase flow is a highly complex process. Multi-scaleand multi-physical components are involved and interrelated, such as the nucleation,growth, departure, coalescence, and collapse of vapour bubbles, turbulence, interfacialinstabilities, a

Evaporative cooling system concepts proposed over the past century for engine thermal management in automotive applications are examined and critically reviewed. The purpose of the review is to establish evident system shortcomings and to identify remaining research questions that need to be addressed to enable this important technology to be adopted by vehicle manufacturers. Initially, the ...