Heat Transfer Enhancement

© 2006, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). (www.ashrae.org). Published in ASHRAE Journal Vol. 48, April 2006. This posting is by  permission of ASHRAE. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE’s prior written permission.

Heat Transfer Enhancement  Heat transer enhancements can improve the heat exchanger eectiveness o internal and external fows. Typically, they increase fuid mixing, by increasing fow vorticity, unsteadiness, or turbulence or by limiting the growth o fuid boundary layers close to the heat transer suraces. By Detlef Westphalen, Ph.D., Member ASHRAE; Kurt Roth, Ph.D., Associate Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

This is the thirty-frst thirty-frs t article inspired by a DOE report covering energy-saving HVAC&R technologies.

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eat exchanger eectiveness can impact the eciency o vapor-compression cycles used in air conditioners, heat pumps, rerigeration equipment, and rerigerators. As the rerigerant condensing or evaporating temperature approaches that o the ultimate heat transer medium, e.g., the outdoor air temperature or an air-cooled condenser, the vapor compression cycle temperature dierence (also reerred to as the lit) decreases. This, in turn, decreases the   pressure ratio across the compressor, increasing its operational coecient o perormance (COP) and decreasing its energy consumption. In vapor compression cycles, enhancement techniques augment both rerigerant- and air-side heat transer. Due to the more avorable heat transer characteristics o rerigerants and  liquids relative to air, and the common use o helical grooves (rifing) to enhance rerigerant-side heat transer, air-side heat transer tends to limit overall heat exchanger eciency, accounting or two-thirds or more o total heat transer resistance. Consequently, this column ocuses primarily on air-side heat transer enhancement techniques. In the absence o enhancement, most HVAC air-to-liquid  heat exchangers have laminar low over suraces due to the small hydraulic diameters o spaces between ns. Heat transer in laminar fows occurs across a thermal boundary layer between the heat exchanger surace and the airfow. Unlike turbulent boundary layers, which have vigorous mixing due to turbulent fow structures that readily transer  heat between the surace and the airfow, calmer laminar    boundary layers have lower heat transer coecients, h . To overcome these limitations, heat transer enhancement approaches augment heat transer by either causing a transi68

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tion to turbulent fow fow,, creating vorticity that increases mixing, or restarting the thermal boundary layer to decrease its thickness. Table 1 describes several dierent types o heat transer enhancement. In some cases, the heat exchanger operating conditions permit the fow to be tripped rom laminar to turbulen t fow i subjected  to a suciently strong perturbation. The surace downstream o fow transition then experiences higher heat transer coecients because most resistance to heat transer occurs across a thin viscous fow layer near the wall instead o across the entire boundary layer. Tripping devices used include surace obstructions (steps, coils, tapes, three-dimensional shapes), surace indentations (cavities, dimples), roughness, as well as upstream turbulence and vorticity vorticity..1 Upstream vorticity does not always cause a fow to become turbulent, but its swirling motions can enhance heat transer by increasing mixing between the air at the heat exchanger surace and the bulk airfow. airfow. Examples include wavy ns, surace winglets, and other elements that protrude rom n suraces suciently to generate vorticity vorticity..1,2,3 At the leading edge o a heat exchanger surace, the thermal  boundary  boundar y layer is thin and poses little resistance to heat transe r. As the length rom the leading edge increases, so does the resistance to heat transer. Some designs interrupt the heat transer  suraces to enable the boundary layer to restart, increasing h. Practical examples o devices used to restart boundary layers include oset strips and louvered ns. 1,2,3 All o the approaches approach es discussed previously are passive, i.e., they do not require additional energy to modiy the fow. On the other hand, most increase the pressure drop o the heat exchanger and, in turn, increase the an energy consumption. Consequently,, their net eectiveness depends upon the balance Consequently  between the reduction in compressor compress or power rom increased heat transer and the increase in an power. power.

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Approach

     e     v       i      s      s      a       P

Enhancement Possible

SuraceInterruptions

Slitsorosetfnsinterrupttheboundarylayer,restartingit,creatingsecondaryows,and/orgeneratingowunsteadiness*

50%–100%

SuraceRoughness

 Acceleratestransitionromlaminarowtoturbulent;alsoincreasesturbulentowheattranser

Upto300%

Ridgesorthree-dimensionalshapes(cube,pyramid,etc.)generatesecondaryorunsteadyows*

50%–500%

SuraceProtuberances

     e     v       i       t      c       A

Description

ForcedFlowUnsteadiness

Suracevibrationorsoundwavesthinsorrestartsboundary layerand/orinducessecondaryows

Small**

Electrohydrodynamic(EHD)

High-voltage(>1kV)appliedtoanelectrodenearaplateinducessecondaryowsinboundarylayer(liquidowsonly)

300%+

BoundaryLayerInjection

Enhancementprimarilyormultiphaseows

BoundaryLayerSuction

Removaloboundarylayerrestartsboundarylayerdownstream

50%–500% Large†

* Heattrans Heattranserde erdecrease creasedinse dinseparate paratedowr dowregion. egion. ** Signifcantenhancementpossibleinliquidows(romcavitation)ornaturalconvection. † See Se eBe Berg rgle les, s,1 199 998. 8.

Table 1: Surace vibrations or sound waves thin or restart boundary layer and/or induce secondary fows.1,5

Active approaches use external energy sources to actively alter the fow. Active approaches can enhance heat transer by one or more o the three mechanisms described previously. previously. For  example, large pressure fuctuations imposed acoustically augment heat transer by increasing fow unsteadiness and, in some instances, inducing laminar-to-turbulent fow transition.1

Energy-Saving Potential Typically, a rerigerant-to-air heat exchanger or an air  conditioner has a saturated rerigerant temperature approximately 10°C to 15°C (18°F to 27°F) higher than the ambient air temperature fowing through the condenser condenser.. By reducing the temperature dierence between the rerigerant and air  temperatures, enhanced heat exchange decreases the over overall all temperature lit o the cycle and increases the cycle’s coecient o perormance (COP). Airside heat transer enhancements have been used or evaporators, although condensation and rost ormation on the heat exchanger surace i o  evaporators complicates the application o enhancements to evaporator evaporators. s. Analyses perormed by TIAX to assess the energy-perormance gains or air-cooled air conditioning and rerigeration cycles indicate a 100% increase in condenser heat transer  coecient can reduce cycle energy consumption by approximately 10% to 15% over a range o conditions. On a national  basis, this could reduce the 7 quads consumed by residential and commercial air conditioners and rerigeration equipment6  by about 0.7 to 1.1 quads. However, this does not account or  additional an energy consumption, which varies on a case by-case basis.

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Market Factors Heat exchanger design and selection cannot be separated  rom a system context and refects several tradeos including : compressor energy eciency gains, heat exchanger airside  pressure drop (an power), component costs, and component size. The extent that enhanced heat exchange aects these design variables as compared with other perormance enhancement options ultimately aects its ability to penetrate the market and appear in products. For example, heat transer  enhancement options may compete with the use o larger  conventional heat exchangers and higher-eciency compressors to increase unit eciency and meet an EER target or  one product. Alternately, enhanced heat exchange may be employed  to decrease heat exchanger area and reduce unit cost or  size while maintaining acceptable energy perormance. For example, heat transer enhancement may prove attractive or certain unitary or military products because they allow the manuacturer to maintain adequate perormance without increasing—or while decreasing—the size o the unit’s enclosure. Manuacturers may be cautious about introducing novel heat transer enhancement approaches into products. I ap proaches require appreciable modications to existing heat exchanger manuacturing processes, manuacturers may have to incur signicant cap ital costs. To To mitigate technical ris ks o  changing heat exchanger geometry, manuacturers must rst  perorm extensive laboratory and eld testing to veriy that the enhancements enhancemen ts perorm reliably over the range o expected  operating conditions. condition s. Notably, Notably, the airside o condensers n eeds

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to resist perormance degradation rom ouling by grease and  dust. This tends to limit the applicability o many potential enhancement approaches.2 Specic heat transer enhancement approaches have their  own challenges to commercialization. For instance, Electrohydrodynamics (EHD) or enhancing water- or rerigerant-side heat transer relies upon high voltages (sometimes in excess o  10 kV) to achieve dramatic (300%+) increases in heat transer  coecients.4,5 Implementing EHD has several complicating actors, including manuacturing complexity comp lexity (to insert the highvoltage electrodes in the tubes), saety concerns, and possible reliability issues.

References 1. Bergles, A.E. 1998. “Techniques “Techniques to enhance heat transer.” Chapter 11 o  Handbook  Handbook o Heat Transer , 3 rd ed. W.M. Rohsenow, J.P. Hartnett, and Y.I. Cho, eds. New York: McGraw-Hill.

3. Gidwani, A., A., M. Molki, and M.M. Ohadi. 2002. “EHDenhanced condensation o alternative rerigerants in smooth and corrugated tubes. tubes.”” HVAC&R Research 8(3). 4. Jacobi, A.M. and R.K. Shah. 1998, “Air-side fow and  heat transer in compact heat exchangers: a discussion o  enhancement mechanisms. mechanisms.””   Heat Transer Engineering , 19(4):29 – 41. 5. TIAX. 2002. “Energy consumption characteristics o  commercial building HVAC systems—Volume III: energy savings potential.” Final Report to U.S. Department o Energy Energy,, Oce o Building Technologies. 6. DOE. 2005. “2005 Buildings Energy Databook.” Pre pared or the U.S. Department o Energy Oce o Energy Eciency and Renewable Energy. http://buildingsdatabook. eren.doe.gov/.

  Detle Westphalen, Ph.D., is principal and Kurt W. Roth,   Ph.D., is associate principal in the HVAC and Rerigeration 2. Bullard, C.W. C.W. and R. Radermacher. 1994. “New technolo- Technology sector sect or o TIAX, Cambridge, Mass. James Brodrick, gies or air conditioning and rerigeration.” rerigeration.” Annual Review o   Ph.D., is a project manager, Building Technologies Program,  pp. 113–152.  Energy and Environment  pp. U.S. Department o Energy, Washington, D.C.

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