Numerical study on heat transfer characteristics in branch tube type ground heat exchanger

Choi, Hoon Ki,Yoo, Geun Jong,Pak, Jae Hun,Lee, Chang Hee Elsevier 2018 Renewable energy Vol.115 No.-

<P><B>Abstract</B></P> <P>A ground heat exchanger is an essential component of ground source heat pumps, which saves cooling and heating energy effectively. In this study, a branch tube type heat exchanger is suggested for a ground heat exchanger instead of commonly using the U-tube type for increasing heat transfer efficiency. The branch tube type heat exchanger has conjugate heat transfer including convective heat transfer between the walls of primary- and branch-tubes and internally circulating fluid, and conduction in the tube walls and grout. This conjugate heat transfer phenomenon is analyzed by steady 3-D numerical analysis using the finite volume method. In the analysis, temperature distribution, rate of heat transfer and pressure drop are compared for the branch tube type ground heat exchanger with 2, 4, 6, and 8 branch tubes together with a single U-tube type ground heat exchanger. Generally, the branch tube type shows better heat transfer performance compared to the U-tube type and the higher number of branch tubes give better heat transfer performance. Also, the branch tube type yields higher pressure drop than the U-tube type for the same mass flow rate of circulating fluid in the heat exchanger.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Branch tube type ground heat exchanger is proposed for enhanced heat transfer. </LI> <LI> Branch tube type ground heat exchanger has increased heat transfer area. </LI> <LI> The increased heat transfer rate is found in the out-flow region. </LI> <LI> Heat transfer rate is affected by number of branch tubes more than inlet velocity. </LI> </UL> </P>

Numerical analysis for the conjugate heat transfer of skin under contrast therapy

Jeon, Byoung jin,Choi, Hyoung Gwon Elsevier 2015 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.86 No.-

<P><B>Abstract</B></P> <P>The effect of natural convection on the temperature distribution of skin was numerically investigated by solving the conjugate heat transfer of the skin under contrast therapy, where hot and cold stimulations are periodically induced on the skin. A finite volume method based on the SIMPLE algorithm was adopted to solve the axisymmetric incompressible Navier–Stokes equations coupled with an energy equation. Those equations were strongly coupled with the Pennes bio-heat equation of skin for the analysis of the conjugate heat transfer. The amplitudes of the sinusoidal temperature profiles of the skin at selected depths were found to become smaller when natural convection was considered, and the temperature evolution obtained by analyzing the conjugate heat transfer differed from that obtained with constant heat transfer coefficients or with a conduction effect only, especially when blood perfusion was included. The spatially-averaged heat transfer coefficient predicted from the present simulation of conjugate heat transfer of contrast therapy was also confirmed to be close to the typical values employed in previous numerical studies. More importantly, the present conjugate heat transfer simulation for contrast therapy revealed that the heat transfer coefficient on skin varies both spatially and temporally. Therefore, the conjugate heat transfer analysis, in which the heat transfer coefficient is calculated as a function of time and space, must be employed in order to accurately predict the temperature evolution inside the skin when subjected to contrast therapy.</P>

Regenerative-Cooling Heat-Transfer Performance of Mg/CO2 Powder Rocket Engines for Mars Missions

Supeng Zhu,Ronggang Wei,Chunbo Hu,Chao Li 한국항공우주학회 2023 International Journal of Aeronautical and Space Sc Vol.24 No.3

Aimed at the problem of thermal protection due to the long-time operation of Mg/CO2 rocket engines under high pressure and heat flux, a regenerative-cooling heat-transfer model is established that considers liquid-CO2 phase change under engine heat flow. How the height-to-width ratio of the cooling channel and the coolant mass flow rate influence the heat-transfer performance is studied, and the results show that the CO2 experiences liquid-phase, two-phase boiling, and gas-phase heat transfer. It is necessary to carry out division-of-region design to avoid membrane-boiling or gas-phase heat transfer in the cooling channel for a long time. When the coolant enters the nuclear boiling stage of two-phase heat transfer near the throat, the heat-transfer coefficient reaches its maximum value, as does the temperature of the corresponding combustor wall, which is 918.1–1012.3 K. When the CO2 flow rate exceeds 5 g/s, the continuous increase of coolant flow has a limited increase in the heat-transfer coefficient of liquid and two-phase heat transfer, but continuing to increase the CO2 flow rate can expand significantly the occurrence region of phase-change heat transfer, so as to bring better heat transfer.

X-ray imaging analysis on behaviors of boiling bubbles in nanofluids

Park, Hanwook,Lee, Sang Joon,Jung, Sung Yong Elsevier 2019 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.128 No.-

<P><B>Abstract</B></P> <P>Nanofluid, a liquid suspension containing nanoparticles, has been widely used to enhance heat transfer. However, the heat transfer enhancement mechanism of nanofluids has not been clearly revealed yet. Therefore, understanding the boiling heat transfer of nanofluids is a challenging research issue in the field of heat transfer. When nanoparticles are added into a base fluid, the thermo-physical properties of the fluid and surface characteristics are modified. Those modifications induce changes in the behavior of boiling bubbles. In addition to the size and number of bubbles, the generating rate of boiling bubbles was newly defined to estimate the heat transfer coefficient directly from bubble behaviors according to nanofluid concentration. As the nanofluid concentration increases, both the generating rate of boiling bubbles and the heat transfer coefficient decrease. Wettability and hydrodynamic size were also employed to reason those degradations. Wettability increase leads to the reduction of activated nucleation sites, which reduces the bubble generating rate and heat transfer coefficient with an increase in nanofluid concentration. In this study, the feasibility and usefulness of synchrotron X-ray imaging and a newly defined boiling generating rate for examining boiling heat transfer were verified by observing boiling-bubble behaviors. Moreover, it was also shown that wettability plays an important role in changes of the bubble behaviors and the heat transfer coefficient as surface roughness modification.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The behavior of boiling bubbles were investigated using a synchrotron X-ray imaging. </LI> <LI> The bubble generating rate was newly defined to examine the nanofluid heat transfer. </LI> <LI> The bubble generating rate decreases with increase of nanofluid concentration. </LI> <LI> Wettability increase reduces the bubble generating rate and heat transfer. </LI> </UL> </P>

Effect of the injection angle on local heat transfer in a showerhead cooling with array impingement jets

Jung, Eui Yeop,Park, Chan Ung,Lee, Dong Hyun,Kim, Kyung Min,Cho, Hyung Hee Elsevier 2018 International journal of thermal sciences Vol.124 No.-

<P><B>Abstract</B></P> <P>An experimental study was conducted to investigate the effect of the injection angle for staggered array impingement jets in a showerhead cooling system. We suggested the angled jets array to enhance heat transfer performance, and heat transfer characteristics were compared to that on the normal jets array. The semicircle test section was designed to consider the showerhead configuration, and the Reynolds number was changed from 3000 to 10,000. The naphthalene sublimation method was used to evaluate the heat transfer coefficients on targeted plates, and the numerical simulations were carried out to analyze the characteristics of the flow fields. The averaged heat transfer coefficient increased monotonically with increasing Re<SUB>d</SUB> due to the enhanced flow mixing by increasing mass flow rate. A low heat transfer coefficient was observed among the adjacent impingement jets in the vicinity of the central row of holes due to staggered array pattern and curvature effect. To overcome this disadvantage, the inclined jets were applied in present study. The heat/mass transfer coefficients were larger for the inclined jets for all Re<SUB>d</SUB> because an amount of wall jet is flowed toward the low heat transfer region by flow imbalance. As applied inclined array impingement jets, averaged heat transfer was enhanced about 9% in low Re<SUB>d</SUB> compared to normal array impingement jets.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Effect of injection angle on impingement jet was analyzed according to Re<SUB>d</SUB> on concave surface. </LI> <LI> Local heat transfer distribution was evaluated using naphthalene sublimation method. </LI> <LI> The inclined array impingement jets enhanced averaged heat transfer about 9% in low Re<SUB>d</SUB> than normal array impingement jets. </LI> </UL> </P>

Heat/mass transfer over the cavity squealer tip equipped with a full coverage winglet in a turbine cascade: Part 1 – Data on the winglet top surface

Joo, Jin Sung,Lee, Sang Woo Elsevier 2017 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.108 No.1

<P><B>Abstract</B></P> <P>Heat/mass transfer characteristics on the winglet top surface for the cavity squealer tip equipped with a full coverage winglet has been investigated experimentally with the variation of <I>h</I>/<I>s</I> (tip gap height-to-span ratio) by employing the naphthalene sublimation technique. For a squealer rim height-to-span ratio of <I>h</I> <SUB>st</SUB>/<I>s</I> =3.75% and a winglet width-to-pitch ratio of <I>w</I>/<I>p</I> =10.55%, the tip gap is changed to be <I>h</I>/<I>s</I> =0.34%, 0.68%, 1.02%, 1.36%, and 1.70%. On the winglet top surface, high local heat/mass transfer rate is found (i) at the sites of the leading edge tip gap vortices, (ii) in the flow separation/reattachment area on the pressure-side winglet, (iii) in the upstream outflow area on the suction-side winglet, (iv) in the downstream outflow area on the suction-side winglet, and (v) in the area where the cavity fluid is discharged toward the trailing edge. On the other hand, very low local heat/mass transfer rate is observed in two separate areas on the suction-side winglet upstream of the mid-chord. In the case of very low <I>h</I>/<I>s</I>, the two separate low heat/mass transfer areas cannot be identified. Instead, a new high heat/mass transfer area is found near the mid-chord on the suction-side winglet. Average heat/mass transfer rate on the winglet top surface tends to increase consistently with increasing <I>h</I>/<I>s</I>, but it is always lower than that on the tip surface of the plane tip with no winglet.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Cavity squealer tip with full coverage winglet is tested in a turbine cascade. </LI> <LI> Effects of tip gap (<I>h</I>) on heat transfer on winglet top surface are presented. </LI> <LI> Local over-tip leakage flow and heat transfer on it are discussed in detail. </LI> <LI> Average heat transfer rate increases consistently with increasing <I>h</I>. </LI> </UL> </P>

Heat Transfer Enhancement for Fin-Tube Heat Exchanger Using Vortex Generators

Yoo, Seong-Yeon,Park, Dong-Seong,Chung, Min-Ho The Korean Society of Mechanical Engineers 2002 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.16 No.1

Vortex generators are fabricated on the fin surface of a fin-tube heat exchanger to augment the convective heat transfer. In addition to horseshoe vortices formed naturally around the tube of the fin-tube heat exchanger, longitudinal vortices are artificially created on the fin surface by vortex generators. The purpose of this study is to investigate the local heat transfer phenomena in the fin-tube heat exchangers with and without vortex generators, and to evaluate the effect of vortices on the heat transfer enhancement. Naphthalene sublimation technique is employed to measure local mass transfer coefficients, then analogy equation between heat and mass transfer is used to calculate heat transfer coefficients. Experiments are performed for the model of fin -circular tube heat exchangers with and without vortex generators, and of fin-flat tube heat exchangers with and without vortex generators. Average heat transfer coefficients of finn-flat tube heat exchanger without vertex generator are much lower than those of fin-circular tube heat exchanger. On the other hand, fin-flat tube heat exchanger with vortex generators has much higher heat transfer value than conventional fin-circular tube heat exchanger At the same time, pressure losses for four types of heat exchanger is measured and compared.

Conjugate heat transfer analysis for the effect of the eccentricity of hydrophobic dot arrays on pool boiling

Lee, Jung Shin,Lee, Joon Sang Elsevier 2017 Applied thermal engineering Vol.110 No.-

<P><B>Abstract</B></P> <P>Patterned wettability causes pinning of the contact line, which is the area responsible for the heat flux transition. A new parameter, “eccentricity,” which governs the contact line shape and length, can be used to control the shape of the bubbles. Initially, we placed four hydrophobic dots on a surface. When a high input heat flux was applied, the bubbles merged and the contact line pinned onto the hydrophobic dots, thus preserving the relatively long dumbbell-shaped contact line. Additionally, the heat transfer coefficient was found to be high at high eccentricity values because of the separation of the merging timing and the generation of metastable contact lines. Next, we placed 36 dots on the surface. The merging and pinning behavior and heat transfer coefficient revealed two stages of transition by the heat flux. From a low to intermediate heat flux, a high eccentricity became effective because the merging and pinning location moved from a marginal area to the entire surface. From the intermediate to high heat flux, large bubbles formed through the simultaneous merging of all the nucleated bubbles, and the eccentricity had almost no influence on the merging or pinning. Therefore, the pinning count minus the number of times the lines merged comprised the relevant data required to analyze the heat transfer coefficient variation with heat flux.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Boiling heat transfer was controlled by varying the eccentricity of the hydrophobic dots. </LI> <LI> Temperature fields of fluid and solid substrate are solved concurrently to consider conjugate heat transfer. </LI> <LI> The contact line of a single bubble could contact several hydrophobic dots to form a twisted contact line. </LI> <LI> The merging and pinning behavior of the bubbles was correlated with the heat transfer coefficient. </LI> <LI> The pinning–merging count showed a similar trend to the heat transfer coefficient. </LI> </UL> </P>

곡률비 변화에 따른 이중관 나선형 열교환기의 열전달 특성에 관한 실험적 연구

금성민,이승로 한국기계기술학회 2023 한국기계기술학회지 Vol.25 No.6

The heat transfer characteristics of double-pipe spiral heat exchanger were investigated by various curvature sizes, experimentally. The three different sizes of heat exchanger were made and tested with water as a working fluid to analyze the heat transfer characteristics. The heat transfer rates, overall heat transfer coefficient and pressure drop were analyzed with various heat exchanger sizes (i.e., curvature ratios). As result, the heat transfer rate increased with increasing the size of the heat exchanger as the flow rate increased due to increasing the area size of heat transfer. However, the overall heat transfer coefficient and pressure drop increased with decreasing the heat exchanger size (i.e., increased curvature ratio) due to the enhanced centrifugal force and inertia.

The heat transfer characteristics of CO₂ and CO₂-oil mixture in tubes

Rin Yun(윤린) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

The heat transfer characteristics of CO₂ and CO₂-oil mixture in tubes including convective flow boiling, gas cooling, and condensation are investigated. Two-phase flow patterns are thoroughly investigated based on physical phenomena, which show the early flow transition to intermittent or annular flow especially for small diameter tube. The physical phenomena for nucleate boiling of CO₂ follow the same trends with other organic fluids under the same reduced pressure. The gas cooling heat transfer is critically dependent on the turbulent diffusivity related with buoyancy force due to the large density difference. Under the oil presence conditions, the interaction of oil rich layer and bubble formation is the physical mechanism for the CO₂-oil mixture convective boiling. Besides, the gas cooling phenomena with oil should be investigated based on the flow patterns formed by CO₂ and oil, and the oil rich layer, whose thickness are depends on the solubility of CO₂ to oil explains the physical mechanisms of heat transfer. The thermodynamic properties of CO₂-oil were estimated by the general model based on EOS, and they are utilized to estimate the properties for oil rich layer and oil droplet vapor core. Through these predicted properties, the convective boiling and gas cooling heat transfer coefficients and pressure drop theoretically estimated. Condensation of CO₂ is not so different from the existing one, so the heat transfer coefficients and pressure drop are well estimated by the existing one developed for other fluids.