Developments in Heat Transfer Part 6 pot

Preparation and thermal properties of ethylene glycole distearate as a novel phase change material for energy storage.. Preparation, characterization, and thermal properties of microenca

wide reaction temperature range, high heat and mass transfer rates, fast reaction kinetic, low material prices, non toxic material Thus, the combination of a porous shell with moisture-sensitive compound as xylitol would be useful for a material design of new functional microparticles for thermal and moisture management (Salaün et al., 2011)

6 Acknowledgment

I would like to thank the French Institute of Textiles and Clothing (IFTH, 2 rue de la Recherche, 59650 Villeneuve d’Ascq, France) and Damartex (2, avenue de la Fosse-aux-Chêne, 59100 Roubaix, France) for funding these researches

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characterization of microencapsulated hexadecane used for thermal energy storage

Chinese Chemical Letters, Vol.15, No.6, (2004), pp 729–732, ISSN 1001-8417

Zuckerman, J.L., Pushaw, R.J., Perry, B.T & Wyner, D.M (1997) Fabric containing anergy

absorbing phase change material and method of manufacturing same US Patent 5,

514,362, available from http://patft.uspto.gov/

Zuckerman, J.L., Pushaw, R.J., Perry, B.T & Wyner, D.M (2001) Fabric coating composition

containing energy absorbing phase change material US Patent 6,207,738, available

from http://patft.uspto.gov/

11

Heat Transfer and Thermal Air Management in the Electronics and Process Industries

Since most enterprise data centres run significant quantities of redundant power and cooling systems to produce higher levels of resiliency, this had led to significant power consumption inefficiencies The latter are exacerbated by the inefficiencies in the Information Technology (IT) hardware and cooling requirements, each accounting for roughly 40% of the total energy usage This results in each KWh of energy for data processing requiring a further KWh for cooling (Almoli et al., 2011) In a typical data centre, electrical energy is drawn from the main grid to power an uninterruptible power supply (UPS) which then powers the IT equipment, supply power to offices and to power the cooling infrastructure: computer room air conditioning (CRAC) units, building chilled water pumps and water refrigeration plant The IT load inefficiencies can be improved by server virtualisation and improved semi-conductor technologies, while the chiller plant is

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generally the biggest energy cooling component and increasing the set point temperature of the chilled water leaving the chiller evaporator offers significant potential reductions in the overall cooling plant energy consumption

Fig 1 A large scale data centre with several rows of server racks

A key strategy for efficient thermal air management in a data centre, as recommended in the

EU Code of Conduct on Data Centre Energy Efficiency, is to separate hot and cold air via a layout of alternating hot and cold aisles (Rasmussen, 2006; Niemann, 2008) These are shown

schematically in Figure 2 In the cold aisle containment strategy, cold air is supplied from the

CRAC units through floor tiles or diffusers into cold aisles and the racks are arranged so that all server fronts/intakes face cold aisles This counteracts the problem that arises if all rows are arranged with intakes facing the same way, when equipment malfunction is

inevitable due to server overheating (Cho et al., 2009) In the hot aisle containment strategy, it

is the hot air that is contained and this approach can have advantages in terms of obviating the need for raised floor tiles and providing hotter air to the CRAC units, increasing their overall efficiency of performance The importance of good air flow management in data centres has led to increasing use of Computational Fluid Dynamics (CFD) (Versteeg & Malalasekera, 1995) to design data centre operations to ensure the thermal environment within data centres conforms to narrow, acceptable bands Care must, however, be taken to ensure that CFD predictions are properly validated and the limitations of its key assumptions (for example on the coupling between the small-scale server air flows and the larger scale data centre air flows) are understood (Almoli et al., 2011) Once validated, CFD models can be very useful for data centre air flow management in enabling a large number

of design scenarios to be investigated and optimal server rack configurations to be identified much more quickly than would be possible experimentally

Heat Transfer and Thermal Air Management in the Electronics and Process Industries 201

Cold Aisle Containment

H H H

C R A C

C R A C

R A C K

R A C K

H C H

C Cplenum

C R A C

R A C K

R A C K

plenum H H

C C

H

Fig 2 Cold and hot aisle containment strategies

Relying on air as the primary heat transfer medium in data centres is becoming increasingly problematical due to inexorable increases in power densities in IT equipment The reduced effectiveness of using air to cool servers is promoting much greater interest in a range of promising alternative technologies based on direct liquid loop cooling, such as dielectric liquid immersion and on-chip spray and jet impingement cooling (Garimella, 2000) This is because the higher heat capacities and associated heat transfer coefficients of liquids mean that they are much more efficient at transferring the waste heat, but with the disadvantages

of requiring liquid loops as close as possible to the heat source Some of the most promising liquid cooling technologies in electronics are discussed briefly in section 2

Energy consumption in the process industries is also currently an area where a significant amount of research is being conducted Due to the enormous range of heat transfer technologies deployed in the process industries, this chapter focuses on one important heat transfer component of several industrial applications, namely the use of convective heat transfer from impinging air jets within industrial ovens (Martin, 1977; Sarkar & Singh, 2004) These are used in applications ranging from the tempering of glass, drying of paper, textiles and precision coated products, to the cooling of metal sheets, turbine blades and, indeed, electronic components, as well as several examples in the food processing and baking industries Forced-convection ovens in the coating, converting and baking industries typically use arrays of hot air impingement jets to transfer heat into products in order to, in the former cases, vaporise their solvent components, and in the latter cases to bake important food products such as bread, see Figure 3

Developments in Heat Transfer

Baking is also a complex process of simultaneous heat, water and water vapour transport within the product where the heat is supplied by a variety of indirect-fired and direct-fired forced convection ovens (Zareifard et al., 2006) Indirect ovens rely on radiation from heated elements within an oven, whereas forced convection ovens are now increasing in popularity since they can offer greater levels of thermal efficiency (Khatir et al., 2011) In the bread baking industry, the primary concern is the effect of heat transfer on the final product quality, which is influenced by the rate and amount of heat application, the temperature uniformity and humidity levels in the baking chamber and the overall baking time (Zhou & Therdthai, 2007) Temperature distribution is particularly important since it affects the enzymatic reaction, volume expansion, gelatinization, protein denaturation, non-enzymatic browning reaction and water migration The timing and application is also very important since supplying too high a temperature can cause early crust formation and a shrunken loaf Forced convection ovens in the baking industry transfer heat to the product by convection from the surrounding air, radiation from the oven walls to the product surfaces and conduction from its containers The relative importance of convection and radiation is determined by the baking temperatures and the speeds of the impinging jets; for low air speeds (~1m/s) radiation is the predominant mode while convection is much more important for higher air speeds (Boulet et al., 2010) Most previous studies in the bread baking industry have tended to focus on regimes with relatively low air speeds, where radiative heat transfer is most influential (Kocer et al., 2007), although high air speeds are now receiving greater attention in the literature

For many years the design and control of baking ovens relied on empirical models, correlating overall performance with simple global parameters such as chamber volume, the

Heat Transfer and Thermal Air Management in the Electronics and Process Industries 203 temperature of the heating elements and inlet conditions (Carvalho & Nogueira, 1997) However, the increasing need to reduce energy consumption during baking has led to far greater use of sophisticated mathematical models in order to optimise baking conditions These include models of the internal temperature and moisture conditions inside the dough/bread (Zheleva & Kambourova, 2005) and several analyses based on Computational Fluid Dynamics, which predict the velocity and temperature distributions within baking chambers Recent studies by Zhou & Therdthai (2007) and Norton & Sun (2007) have shown how a baking oven’s energy consumption can be reduced by manipulating airflow patterns

so as to increase the volume of airflow while reducing the energy supplied CFD models can also provide valuable insight into key baking issues that influence product quality, such as temperature uniformity, that are difficult to measure experimentally

This chapter presents a brief review of some of the key thermal management challenges in the electronic and process industries that are being addressed by current research projects both at the University of Leeds and at other institutions In the electronics industry, the focus is on the rapidly burgeoning data centres industry, where efficient thermal air management is crucial The current role, capabilities and limitations of CFD modelling in this sector are discussed, as are the promising future liquid cooling technologies that will be increasingly needed as the limits of air cooling methods are reached In the process industries, the particular focus is on the challenges of improving the energy efficiency of forced convection ovens used throughout the coating, converting and bread baking industries The key role of CFD modelling in improving oven design and operation is discussed, together with a brief overview of the future experimental and computational research needed to embed computational design methods into industrial practice

2 Heat transfer and thermal airflow management in data centres

2.1 Air cooling management

As discussed above, currently most data centre cooling is achieved using cold air supplied

by CRAC units into data centres through raised floor tiles that then passes through the server racks, cools the electronic equipment and emerges from the back of the servers as a hot air stream, see Figure 2 Maintaining temperature and humidity design conditions is critical to the good operation of data centres and generally temperature conditions at the inlet to the racks should be maintained between 20-30oC and 40-55% relative humidity in order to prevent equipment malfunction (Cho et al., 2009) Recent figures from the ASHRAE trends in rack heat load shows typical server heat fluxes of 27KW for a 19 inch rack (Shrivastava et al., 2009) and these will be even larger today

The European Commission has created an EU Code of Conduct in response to increasing energy consumption in data centres and the need to reduce the related environmental, economic and energy supply security impacts The Code of Conduct aims to achieve this by improving understanding of energy demand within a data centre, raising awareness and recommending energy efficiency best practice and targets The Code of Conduct makes several important recommendations for air flow management in data centres in order to improve overall energy efficiency A key recommendation is that the hot/cold aisle layout should be implemented which aims to minimise the amount of bypass air, which returns to CRAC units without performing cooling, and the amount of mixing of cold and hot air which leads to higher air intake temperatures into servers As shown in Figure 2 the hot/cold aisle concept aligns equipment airflow to create aisles between racks that are fed

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cold air from which the electronic equipment draws intake air in conjunction with hot aisles

to which all equipment exhausts hot air

Although the cold air containment strategy is probably the most common today, the

alternative approach, termed hot-aisle containment, is also increasing in popularity (Niemann,

2008) In this approach the hot air from the servers is contained and is cooled before being recirculated back into the room Key advantages of this approach that have been proposed include:

• it does not impact on surrounding data centre infrastructure and obviates the need for raised floor tiles

• it enables return air to be returned to CRAC units at higher temperatures, enabling the chillers to operate more efficiently and increase the proportion of the year during which free cooling technologies (where no compressor is required) can be utilised

• reduced humidification and de-humidification costs, saving energy and water

There are currently conflicting opinions about which containment strategy is the best in practice, however maximising the use of free cooling is another key recommendation of the

EU Code of Conduct Other key thermal air management recommendations of the Code of Conduct include:

• the use of blanking plates where there is no electronic equipment in order to prevent cold air passing through gaps in the rack;

• installing aperture brushes to cover all air leakage opportunities provided by floor openings at the base of racks and gaps in their sides;

• use of overhead cabling to prevent obstructions in air flow paths that increase the fan power needed to circulate air throughout the data centre

In addition to encouraging imaginative use of the waste heat produced in data centres, such

as using the low grade heat for buildings and swimming pools, the ability to control the thermal air environment in data centres more accurately enables the chilled water set point temperature to be increased, maximising the use of free cooling and reducing compressor energy consumption significantly

2.2 CFD modelling of thermal air flows in data centres

Computational Fluid Dynamics (CFD) is now frequently used to design the layout of servers within data centres Thermal air flows in data centres are complex, recirculating air flows characterised by multiple length scales, modes of heat transfer and flow regimes Length scales range from processor length scales (order of mm) to rack length scales (of the order of metres) up to data centre length scales (order of several metres) A typical Reynolds number,

Re, based on a typical air inlet velocity from supply vents of 1 m/s and a rack length scale of 2m leads to an estimated Re≈105 indicating the turbulent flow regime (Almoli et al., 2011) However, as discussed by Choi et al (2008), for the flow through servers racks the Reynolds numbers are typically much smaller and may even lie within the challenging laminar-turbulent transition regime which requires different flow models from those that can be used at the data centre length scales for fully developed turbulent conditions At present there is no effective multi-scale CFD model that integrates the thermal circuit modelling of microprocessors and data centre scale thermal flow modelling and which is capable of adapting to dynamic conditions within data centres

However, most previous CFD studies of data centre airflows have simply assumed the flow outside the racks is fully turbulent and have used Reynolds Averaged Navier Stokes (RANS) flow models, see e.g Cho et al (2009), while modelling the racks in a compact

Heat Transfer and Thermal Air Management in the Electronics and Process Industries 205

manner without explicit representations of internal components These are based on the

following governing continuity and momentum equations, written in RANS format as

where σ=−P I+μ(∇ + ∇U ( U)T) is the Newtonian stress tensor, µ is the air viscosity, ρ its

density, U and U’ are the average and turbulent fluctuation velocity vectors respectively, P

is the pressure and I the unit tensor The vector S represents the additional momentum

sources, which are discussed below, and the −ρU U' 'term is the Reynolds stress tensor that

requires additional model equations

Most CFD models of data centre airflows use the standard k-ε model (Cho et al., 2009)

where the turbulence is modelled in terms of the turbulent kinetic energy (k) and turbulent

dissipation (ε) The two additional transport equations for the k-ε model are:

ij ij k

ij ij C

ε

ε ε

ε μμ

= where

p

k C

αρ

k is the thermal conductivity and Cp is the air’s specific heat capacity The subscript T

indicates the turbulent flow and SQ is the source term of the energy equation, namely the

heat generated by the processors

Several commercial CFD codes have now been used to solve air flows in data centres,

ranging from general purpose codes such as Ansys Fluent 12 (Almoli et al., 2011), to a

number of codes specifically developed for the rapidly growing data centre industry; the

latter include CFD software packages such as Flovent, Six Sigma and TileFlow which are

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206

designed for maximum ease of use However, it is important to recognize that CFD is still largely unverified for data centre airflows (Shrivastava et al., 2009), and that a hierarchy of models is required for the data centre air flows and air flows through the racks All CFD models of data centre air flows should ideally only be used after careful validation against experimental data

The recent study by Almoli et al (2011) noted that previous CFD studies of data centre air flows have provided very little explanation of the way the flow through server racks are modelled This makes it very difficult to carry out meaningful comparisons with previous CFD studies They proposed that an efficient coupling between the data centre air flows and air flow through the racks could be achieved by treating the racks as porous media Their permeabilities can be estimated experimentally by measuring pressure drops across the rack for a range of flow rates and the rate of heat generation by the IT equipment can be estimated from manufacturer’s specifications They used this approach to develop the first CFD model for data centre cooling scenarios where a liquid loop heat exchanger is attached

at the rear of server racks (back doors) which can avoid the need to separate the cold and hot air streams in traditional hot/cold aisle arrangements and can also significantly reduce the load on the CRAC units This study also investigated the effectiveness of additional fans

in the back door heat exchangers

2.3 Alternative liquid cooling techniques

Relying solely on air as the primary heat transfer medium in data centres is becoming increasingly problematical due to inexorable increases in power densities from IT equipment Since liquids have much higher heat capacities and heat transfer coefficients than gases, liquid cooling can potentially be much more effective than gas cooling for high power electronic components However, until relatively recently problems with liquid cooling systems due to leakage corrosion, extra weight and condensation have limited their use to high power density situations where air cooling is simply not viable As discussed in the recent review by Anandan & Ramalingham (2008), a range of alternative liquid cooling technologies are now beginning to be taken up within industry A selection of some of the most promising approaches is outlined briefly below

2.3.1 Dielectric liquid immersion cooling

Here, electronic components are immersed in a dielectric fluid as shown schematically in Figure 4 This involves the boiling of the working fluid on a heated surface and is highly effective since the phase change from liquid to vapour increases the heat flux from the heated surface significantly and the high thermal conductivity of the liquid increases the accompanying convection The main limitation of using these methods is the lack of suitable dielectric fluids, which are usually refrigerant-type fluids whose effectiveness can be limited

by problems associated with the long term corrosion of computer components

2.3.2 Spray cooling

In spray cooling, a cooling agent in form of jet of liquid droplets, is injected through nozzles onto the electronic module The spray is formed by a pressure drop across the nozzle, impinges on the surface and forms a thin liquid film The heat from the electronic module is dissipated by evaporating the cooling agent The resultant hot liquid and vapour is recycled through a spray drain, as indicated in Figure 5

Heat Transfer and Thermal Air Management in the Electronics and Process Industries 207

Pressure reliefvalve

Liquid reservoir

Saftey valve

vapour

Dielectricfluid

Fig 4 Direct liquid immersion cooling

Fig 5 Schematic diagram of the spray cooling approach

Spray cooling is a very promising cooling method for high heat flux applications (Mudawar, 2001) It has specific advantages since spraying the heat source directly eliminates the thermal resistance of the bonding layer in electronic equipment and offers attractive ratios of power supplied for cooling to rate of heat removal An important limitation to the wider adoption of spray cooling is that these must be non-conducting, dielectric liquids Water is often used when a thin protective, coated layer is applied to electronic equipment to reduce the risk of short circuits due to water’s low dielectric strength Relatively few alternative liquids have demonstrated their suitability for spray cooling applications (Chow et al., 1997)

2.3.3 Indirect liquid cooling

As the name suggests, in indirect cooling the liquid cooling agent does not have direct contact with the electronic module and instead a thermal pathway is formed between the module and the cooling agent, as shown in Figure 6 The thermal pathway is often a cold

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plate with high thermal conductivity and since there is no contact between the module and cooling agent, the latter can be any suitable liquid The high thermal conductivity and environmental-friendliness of water make it the most common cooling agent, however foam-filled cold plates are increasingly being used for high heat flux cooling applications (Apollonov, 1999, 2000)

Air to waterheat exchanger

Filter

PumpWater

reservoir

Cold plateElectronic module

Fig 6 Indirect cooling of electronic modules

2.3.4 Liquid jet impingement cooling

Many applications in industry require localised cooling and use impinging liquid jets to achieve this objective Important examples include the cooling of metal sheets, turbine blades and high power density electronic components In electronic cooling, cold liquid jets are typically directed towards a surface from which heat needs to be removed Figure 7 shows schematic diagrams of common approaches to electronic cooling using impinging liquid jets (Anandan & Ramalingham, 2008) These can be classified into free-surface, submerged and confined submerged jets (Wolf et al., 1993)

Nozzle

Gas Liquid

Liquid

Liquid

Gas

Gas Nozzle