• Title/Summary/Keyword: Non-uniform heat distribution

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Experimental study of bubble flow behavior during flow instability under uniform and non-uniform transverse heat distribution

  • Al-Yahia, Omar S.;Yoon, Ho Joon;Jo, Daeseong
    • Nuclear Engineering and Technology
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    • v.52 no.12
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    • pp.2771-2788
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    • 2020
  • Experiments are conducted to study bubble flow behavior during the instability of subcooled boiling under uniform and non-uniform transverse heating. The non-uniform heat distribution introduces nonuniform bubble generation and condensation rates on the heated surface, which is different from the uniform heating. These bubble generation and condensation characteristics introduce a non-uniform local pressure distribution in the transverse direction, which creates an extra non-uniform pressure on the flowing bubbles. Therefore, different bubble flow behavior can be observed between uniform and non-uniform heating conditions. In the uniform heating, bubble velocity fluctuations are low, and the bubbles travel straight along the axial direction. In the non-uniform heating, more fluctuation in the bubble velocity occurs at low mass flow rate and high subcooled inlet temperatures, and reverse flow is observed. Additionally, the bubbles show a zigzag trajectory when they pass through the channel, which indicates the existence of cross flow in the transverse direction.

Computer Simulation Study of the Thermoelectric Cooling by Hybrid Method (하이브리드법을 이용한 열전냉각의 수치해석 연구)

  • Kim, N.J.;Lee, J.Y.;Kim, C.B.
    • Solar Energy
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    • v.20 no.1
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    • pp.97-108
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    • 2000
  • The purpose of this study is to minimize the heat transfer surface area and cold fluid exit temperature of heat exchanger which applied to the refrigeration and air-conditioning system by utilizing the thermoelectric principle. Both uniform and non-uniform current distribution methods which applied to the analysis of the TE elements that incorporates heat exchanger were investigated. The non-uniform current distribution method had the better coefficient of performance and had the lower cold fluid exit temperature of the TE cooling system than the uniform current distribution method. It was found that if a TE cooling system incorporates a heat exchanger, a non-uniform current distribution should guarantee to the lowest cold fluid exit temperature. Also, the hybrid method (combination of the uniform and non-uniform current distribution method) is investigated to achieve the best results by combining the uniform and non-uniform current distributions. The results show that it can lower the cold fluid exit temperature and reduce the heat transfer surface area for the parallel flow arrangement if we apply the constant current in some entry region and the non-uniform increasing current in the direction of the cold fluid flow afterwards.

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An Experimental Study of Critical Heat Flux in Non-uniformly Heated Vertical Annulus under Low Flow Conditions

  • Chun, Se-Young;Moon, Sang-Ki;Baek, Won-Pil;Chung, Moon-Ki;Masanori Aritomi
    • Journal of Mechanical Science and Technology
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    • v.17 no.8
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    • pp.1171-1184
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    • 2003
  • An experimental study on critical heat flux (CHF) has been performed in an internally heated vertical annulus with non-uniform heating. The CHF data for the chopped cosine heat flux have been compared with those for uniform heat flux obtained from the previous study of the authors, in order to investigate the effect of axial heat flux distribution on CHF. The local CHF with the parameters such as mass flux and critical quality shows an irregular behavior. However, the total critical power with mass flux and the average CHF with critical quality are represented by a unique curve without the irregularity. The effect of the heat flux distribution on CHF is large at low pressure conditions but becomes rapidly smaller as the pressure increases. The relationship between the critical quality and the boiling length is represented by a single curve, independent of the axial heat flux distribution. For non-uniform axial heat flux distribution, the prediction results from Doerffer et al.'s and Bowling's CHF correlations have considerably large errors, compared to the prediction for uniform heat flux distribution.

Heat Transfer Performance Variation of Condenser due to Non-uniform Air Flow (불균일한 풍속분포에 따른 응축기의 열전달 성능 변화)

  • Lee, Won-Jong;Jeong, Ji Hwan
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.26 no.4
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    • pp.193-198
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    • 2014
  • Heat transfer performance variation of a condenser caused by non-uniform distribution of air flow was investigated using a numerical simulation method. A heat exchanger used for a outdoor unit of a commercial heat pump system and represented by a numerical model was selected. Non-uniform profile of air-velocity was constructed by measuring the air velocity at various locations of the outdoor unit. Simulation was conducted for various refrigerant circuits and air flow conditions. Simulation results show that the heat transfer capacity was reduced depending on the air-flow rate and the refrigerant circuit configuration. It is also shown that the capacity reduction rate is increased as the average air velocity decreases.

Characteristics of Frost Formed on Heat Exchanger Fins of Non-Uniform Temperature Distribution (불균일한 온도분포를 갖는 열교환기 휜에서의 착상 특성)

  • An, Won-Jun;Kim, Jung-Soo;Lee, Kwan-Soo
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.21 no.7
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    • pp.373-379
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    • 2009
  • In this study, frosting experiment was conducted to investigate the characteristics of frost formed on heat exchanger fins of non-uniform temperature distribution. Temperature distribution and frost characteristics of a 2-D fin surface were investigated in the airflow direction and the direction perpendicular to airflow. Temperature gradient was very small in the airflow direction, while it was large in the direction perpendicular to airflow due to fin heat conduction. The variations of the frost thickness gradient and the frost density gradient in the direction perpendicular to airflow were significant. On the other hand, the temperature gradient on frost surface in the direction perpendicular to airflow was significant at the early stage of frosting, while it decreased gradually as time elapsed.

Conjugated heat transfer on convection heat transfer from a circular tube in cross flow (원관 주위의 대류 열전달에 대한 복합 열전달)

  • 이승홍;이억수;정은행
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.10 no.5
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    • pp.523-534
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    • 1998
  • The convection heat transfer on horizontal circular tube is studied as a conjugated heat transfer problem. With uniform heat generation in a cylindrical heater placed in a cross flow boundary condition, heat flow that is conducted along the wall of the heater creates a non-isothermal surface temperature and non-uniform heat flux distribution. In the present investigation, the effects of circumferential wall heat conduction on convection heat transfer are investigated for the case of forced convection around horizontal circular tube in cross flow of air and water. Non-dimensional conjugation parameter $ K^*$ which can be deduced from the governing energy differential equation should be used to express the effect of circumferential wall heat conduction. Two-dimensional temperature distribution$ T({\gamma,\theta})$ is presented. The influence of circumferential wall heat conduction is demonstrated on graph of local Nusselt number.

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Effect of Circumferential Wall Heat Conduction on Boundary Conditions for Convection Heat Transfer from a Circular Tube in Cross Flow (원관 주위의 대류 열전달에서 경계조건에 대한 원주방향 열전도의 영향)

  • 이상봉;이억수;김시영
    • Journal of Ocean Engineering and Technology
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    • v.15 no.2
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    • pp.39-45
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    • 2001
  • With uniform heat generation from the inner surface of the cylindrical heater placed in a cross flow boundary condition, heat flow that is conducted along the wall of the heater creates a non-isothermal surface temperature and non-uniform heat flux distribution. In the present investigation, the effects of circumferential wall heat conduction on convection heat transfer is investigated for the case of forced convection around horizontal circular tube in cross flow of air. The wall conduction number which can be deduced from the governing energy equation should be used to express the effect of circumferential wall heat conduction. It is demonstrated that the circumferential wall heat conduction influences local Nusselt numbers of one-dimensional and two-dimensional solutions.

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A Method for Critical Heat Flux Prediction in Vertical Round Tubes with Axially Non-uniform Heat Flux Profile

  • Shim, Jae-Woo
    • Journal of Ocean Engineering and Technology
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    • v.22 no.1
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    • pp.13-21
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    • 2008
  • In this study a method to predict CHF(Critical heat flux) in vertical round tubes with axially non-uniform cosine heat flux distribution for water was examined. For this purpose a local condition hypothesis based CHF prediction correlation for uniform heat flux in vertical round tubes for water was developed from 9,366 CHF data points. The local correlation consisted of 4 local condition variables: the system pressure(P), tube diameter(D), mass flux of water(G), and 'true mass quality' of vapor($X_t$). The CHF data points used were collected from 13 different published sources having the following operation ranges: 1.01 ${\leq}$ P (pressure) ${\leq}$ 206.79 bar, 9.92${\leq}$ G (mass flux) ${\leq}$ 18,619.39 $kg/m^2s$, 0.00102 ${\leq}$ D(diameter) ${\leq}$ 0.04468 m, 0.0254${\leq}$ L (length) ${\leq}$ 4.966 m, 0.11 ${\leq}$ qc (CHF) ${\leq}$ 21.41 $MVW/m^2$, and -0.87 ${\leq}X_c$ (exit qualities) ${\leq}$ 1.58. The result of this work showed that a uniform CHF correlation can be easily extended to predict CHF in axially non-uniform heat flux heater. In addition, the location of the CHF in axially non-uniform tube can also be determined. The local uniform correlation predicted CHF in tubes with axially cosine heat flux profile within the root mean square error of 12.42% and average error of 1.06% for 297 CHF data points collected from 5 different published sources.

Behavior of frost formed on heat exchanger fins (열교환기 휜에서의 착상 거동)

  • Kim, Jung-Soo;Lee, Kwan-Soo
    • Proceedings of the KSME Conference
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    • 2008.11b
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    • pp.2334-2339
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    • 2008
  • This paper proposes an improved mathematical model for predicting the frosting behavior on a two-dimensional fin considering the heat conduction of heat exchanger fins under frosting conditions. The model consists of laminar flow equation in airflow, diffusion equation of water vapor for frost layer, and heat conduction equation in fin, and these are coupled together. In this model, the change in three-dimensional airside airflow caused by frost growth is accounted for. The fin surface temperature increased toward the fin tip due to the fin heat conduction. On the contrary, the temperature gradient in the airflow direction(x-dir.) is small throughout the entire fin. The frost thickness in the direction perpendicular to airflow, i.e. z-dir., decreases exponentially toward the fin tip due to non-uniform temperature distribution. The rate of decrease of heat transfer in the airflow direction is high compared to that in the z-direction due to more decrease in the sensible and latent heat rate in x-direction.

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