Performance of a Closed-Tube Thermosyphon With Large Length-Diameter Ratios

1989 ◽  
Vol 111 (1) ◽  
pp. 22-28 ◽  
Author(s):  
G. S. H. Lock ◽  
G. A. Simpson
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Single Phase ◽  
Diameter Ratio ◽  
Lower Section ◽  
Momentum Exchange ◽  
Transfer Data ◽  
Usual Form ◽  
Closed Tube ◽  
Large Length

The experimental work reported here is concerned with the single-phase closed thermosyphon under conditions which are appropriate to a northern application: large length-diameter ratios and heated lengths much greater than cooled lengths. The purpose is twofold: to generate empirical heat transfer data which represent the conservative lower limit of performance (in the absence of boiling or bubbling); and to reveal the thermal and momentum exchange mechanisms which operate at the junction of the heated and cooled sections. The apparatus consisted of a 102-mm-dia, vertical steel pipe of overall length varying from 2 m to 6 m. The lower section of the pipe was heated by means of electric-resistive tape, and the upper section cooled by means of a water jacket. Heat transfer data, plotted in the usual form of Nusselt number versus the quotient of the Rayleigh number and the length-diameter ratio, show the effect of geometry. This data was obtained for 10 ≤ LH/d ≤ 50, and 1 ≤ LH/Lc ≤ 20. An empirical correlation is developed.

Performance of a Closed-Tube Aerosyphon With Large Length-Diameter Ratios

1989 ◽  
Vol 111 (4) ◽  
pp. 337-343
Author(s):  
G. S. H. Lock ◽  
J. D. Kirchner
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Single Phase ◽  
Diameter Ratio ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
General Terms ◽  
Closed Tube ◽  
Order Of Magnitude ◽  
Large Length

The paper reports an experimental investigation of heat transfer in the closed-tube aerosyphon (aerated-thermosyphon) for a range of conditions representative of northern field applications. In particular, attention is focused on the effect of using tubes with heated lengths not only greater than the cooled lengths, but very much greater than the tube diameter. Using three heated sections and one cooled section, the geometry of the device has been varied systematically with 10 < LH/d < 50 and 1 < LH/LC < 20. For any given geometry, the effect of air bubbling rate has been studied in the range of 0 < V˙ < 5 × 10−5 m3/S. Using these ranges it has been possible to make comparisons with other thermosyphon and aerosyphon data. The results indicate that heat transfer coefficients are reduced by increasing either length-diameter ratio or heated-cooled length ratio. They also reveal that, in general terms, the aerosyphon is almost an order-of-magnitude more effective than the single-phase thermosyphon. Some obervations on the flow regimes are offered, and an empirical correlation is presented.

Local Heat Transfer Coefficients Under an Axisymmetric, Single-Phase Liquid Jet

1991 ◽  
Vol 113 (1) ◽  
pp. 71-78 ◽  
Author(s):  
J. Stevens ◽  
B. W. Webb
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Local Nusselt Number ◽  
Single Phase ◽  
Radial Distance ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Plate Spacing

The purpose of this investigation was to characterize local heat transfer coefficients for round, single-phase free liquid jets impinging normally against a flat uniform heat flux surface. The problem parameters investigated were jet Reynolds number Re, nozzle-to-plate spacing z, and jet diameter d. A region of near-constant Nusselt number was observed for the region bounded by 0≤r/d≤0.75, where r is the radial distance from the impingement point. The local Nusselt number profiles exhibited a sharp drop for r/d > 0.75, followed by an inflection and a slower decrease there-after. Increasing the nozzle-to-plate spacing generally decreased the heat transfer slightly. The local Nusselt number characteristics were found to be dependent on nozzle diameter. This was explained by the influence of the free-stream velocity gradient on local heat transfer, as predicted in the classical analysis of infinite jet stagnation flow and heat transfer. Correlations for local and average Nusselt numbers reveal an approximate Nusselt number dependence on Re1/3.

Bubble Injection to Enhance Heat Transfer in Microchannel Heat Sinks

2009 ◽  
Author(s):  
Amy Rachel Betz ◽  
Daniel Attinger
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Single Phase ◽  
Heat Fluxes ◽  
Heat Sinks ◽  
Segmented Flow ◽  
Liquid Cooling ◽  
Microchannel Heat Sinks ◽  
Convective Resistance ◽  
Parallel Microchannels

Liquid cooling is an efficient way to remove heat fluxes with magnitude of 1 to 10,000 W/cm2. One limitation of current single-phase microchannel heat sinks is the relatively low Nusselt number, because of laminar flow. In this work, we experimentally investigate how to enhance the Nusselt number in the laminar regime with the periodic injection of non-condensable bubbles in a water-filled array of microchannels in a segmented flow pattern. We designed a polycarbonate heat sink consisting of an array of parallel microchannels with a low ratio of heat to convective resistance, to facilitate the measurement of the Nusselt Number. Our heat transfer and pressure drop measurements are in good agreement with existing correlations, and show that the Nusselt number of a segmented flow is increased by more than a hundred percent over single-phase flow provided the mass velocity is within a given range.

Local Single-Phase Heat Transfer Research of Natural Circulation in Narrow Rectangular Channel

2017 ◽  
Author(s):  
Zhiqiang Zhu ◽  
Xiaxin Cao ◽  
Changqi Yan ◽  
Chunping Tian
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Inclination Angle ◽  
Local Nusselt Number ◽  
Single Phase ◽  
Natural Circulation ◽  
Rectangular Channel ◽  
Inlet Temperature

In order to explore and analyze the heat transfer characteristics in narrow rectangular channel, experiments on local single-phase heat transfer of natural circulation in a one-side heating narrow rectangular channel have been conducted under vertical and inclined condition. The thermotechnical parameters such as inlet temperature, heat flux and inclination angle varies during the experiments. The width of the flow channel is 40 mm and the narrow gap is 2 mm. It is heated from one side with a homogeneous and constant heat flux and the working medium is deionized water. Based on the experimental results, under vertical condition, the driving force in the loop goes up and the Reynolds number also increases when the inlet temperature is elevated, which causes an increase in local Nusselt number. When the heat flux rises, the local Nusselt number increases and the heat transfer temperature difference increases. The local Nusselts number is influenced by entrance effect and the entrance region length is computed for laminar and turbulent flow. Under inclined condition, with the inclination angle from −30° to 30°, it is found that when the inclination angle is positive, the local Nusselt number in fully developed region is larger than that under vertical condition and increases with the angle value, even though the Reynolds number decreases by the effect of incline. This phenomenon is explained by giving an analysis of the natural convection, which is characterized by the normal Grashof number, in the direction perpendicular to the heating plat. Moreover, the variation of heat transfer is also interpreted on the basis of field coordination principle. However, when the inclination angle is negative, the heat transfer shows no obvious difference between vertical condition and inclined condition.

scholarly journals Experimental investigation of the effect of wave turbulators on heat transfer in pipes

2021 ◽  
pp. 183-183
Author(s):  
Sendogan Karagoz ◽  
Semih Erzincanli ◽  
Orhan Yildirim ◽  
Ilker Firat ◽  
Mehmet Kaya ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Friction Factor ◽  
Enhancement Factor ◽  
Single Phase ◽  
Reynolds Numbers ◽  
Nusselt Numbers ◽  
Thermal Enhancement ◽  
Enhancement Factors ◽  
Thermal Enhancement Factor

This experimental study deals with the heat transfer and friction effects of sinusoidal part turbulators for single-phase flows occurring in a circular shaped pipe. Turbulators with three different radius values are placed in the pipe to make the flow turbulent. In this way, changes in Nusselt number and friction coefficient are examined. As a result of the experiments made with Reynolds numbers in the range of 6614-20710, the increase rates of the Nusselt numbers of turbulators with 20 mm, 110 mm and 220 mm radius compared to the empty pipe were obtained as 153.49%, 85.36%, and 52.09%, respectively. As a result of the decrease in the radius, there was an increase in the Nusselt number and the friction factor. Parallel to the Nusselt number, the highest friction factor was obtained in the smallest radius turbulator. It was found that the thermal enhancement factors of 110 mm and 220 mm radius turbulators increased by 179.54% and 132.95%, respectively, compared to the 20 mm radius turbulator. Similarly, it was determined that the thermal enhancement factor of the 110 mm radius turbulator increased by 20% compared to the 220 mm radius turbulator.

Study of Supersonic Micro-Channel for Cooling Electronic Devices

2013 ◽  
Author(s):  
Yuya Takahashi ◽  
Junnosuke Okajima ◽  
Yuka Iga ◽  
Atsuki Komiya ◽  
Wu-Shung Fu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Gas Flow ◽  
Single Phase ◽  
Laval Nozzle ◽  
Cooling System ◽  
Supersonic Nozzle ◽  
Cooling Performance ◽  
Micro Scale ◽  
Mean Temperature

In this study, we focus on a micro-scale cooling device using a supersonic single phase gas flow. The single phase gas cooling system has advantages for cooling electronics device in a micro-scale. Generally, the forced convective heat transfer by single phase gas flow has a lower heat transfer coefficient than other heat transfer mechanisms. However, the heat transfer rate can be largely improved with a low temperature flow that is generated by isentropic expansion in supersonic nozzle. The objective of this study is to conduct a numerical evaluation of the possibility of this cooling system with a supersonic air flow through a heated micro-fin array. In order to calculate the supersonic flow inside the nozzle and evaluate the effect of the nozzle shape on the heat transfer, two types of nozzles are designed. One nozzle is a typical supersonic nozzle called Laval nozzle. The other is named Bump nozzle which has a simple arc shape at the throat. The channel size of both nozzles are about 200 μm in width and 2743.1 μm in length. In order to estimate the cooling performance, the numerical simulations were conducted by using ANSYS FLUENT 12.1 with the density-based Roe-FDS method. The inlet pressure, outlet pressure, and total pressure were set to 290 kPa, 100 kPa, and 367.1 kPa, respectively. The stagnation temperature and wall temperature were assumed 300 K and 350 K, respectively. The values of bulk mean temperature and Nusselt number were estimated. In both nozzles, the calculated bulk mean temperature was about 230 K and the Nusselt number was 7.54, which is the theoretical value of laminar forced convection between the parallel plates. The results showed that the Bump nozzle had almost the same cooling performance as the Laval nozzle in spite of its simple geometry in the each single channel. In addition, the Bump nozzle can have 4 times the number of channels the Laval nozzle configuration can occupy the same area because of its shape. This indicates that cooling performance of a device that includes the Bump nozzle geometry is higher than that of the Laval nozzle.

Heat Transfer Augmentation Using an Innovative Helicoidal Finned Absorber Plate in a Solar Air Heater—A Numerical Study

2018 ◽  
Vol 141 (3) ◽  
Author(s):  
Shantanu Purohit ◽  
N. Madhwesh ◽  
K. Vasudeva Karanth ◽  
N. Yagnesh Sharma
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Friction Factor ◽  
Diameter Ratio ◽  
Wire Diameter ◽  
Solar Air Heater ◽  
Absorber Plate ◽  
Air Heater ◽  
Pitch Ratio

This study presents an innovative idea to augment heat transfer to an air heater using helicoidal finned arrangement. A parametric analysis of the helicoidal shaped fin geometry is considered with helicoidal pitch ratio of 0.1666–0.3, fin diameter ratio of 1.75–2. For the placement of the fin beneath the absorber plate, longitudinal pitch ratio ranging from 0.0416 to 0.1666 are used. The flow Reynolds number used for the study ranges from 4800 to 25,000. The effects of helicoidal pitch ratio, wire diameter ratio and longitudinal pitch ratio on Nusselt number and friction factor have been discussed. It is seen from the analysis that there is a significant improvement in Nusselt number for the case of helicoidal fin of wire diameter ratio of 1 when compared to base model as well as straight fin model for the operating range of Reynolds number. It is also observed from the analysis that for the helicoidal fin configuration of helicoidal pitch ratio of 0.2333, friction factor appears to be moderate. Flow and roughness parameters for roughened solar air heater have been optimized using thermal-hydraulic enhancement factor (THEF). The study reveals that by the use of helicoidal fins, maximum enhancement in the Nusselt number is found to be 2.21 times when compared to the base model for longitudinal pitch ratio of 0.0416, helicoidal pitch ratio of 0.166 for a fixed wire diameter. The improvement obtained in performance corresponding to increased Nusselt number establishes the efficacy the helicoidal fin design for the absorber plate.

Effects of Surface Roughness on Single Phase Fluid Flow and Heat Transfer Characteristics in Microchannels

2009 ◽  
Author(s):  
Jinli Lu ◽  
Yingli Hao
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Pressure Drop ◽  
Velocity Distribution ◽  
Shape Factor ◽  
Single Phase ◽  
Peak Position ◽  
Centerline Velocity ◽  
Heat Transfer Characteristics ◽  
Flow And Heat Transfer

A two dimensional numerical simulation is conducted to investigate the flow and heat transfer characteristics of single phase liquid laminar flow through rough microchannels. The wall roughness is simulated in a series of cases with rectangular, triangular and trapezoidal elements, respectively. Shape factor and peak position have been used to analyze the influence of roughness elements on centerline velocity distribution, pressure drop and Nusselt number. It is found that the shape factor has a significant effect on the centerline velocity distribution, pressure drop and Nusselt number. It is also found that, for a given shape factor, the effect of peak position on pressure drop is strongly than centerline velocity distribution and heat transfer. In addition, for all considered roughness element shapes, the rectangular element displays a poor heat transfer and large pressure drop.

Experimental Study on Thermal-Hydraulic Performance of Single-Phase Water Flow in Narrow Rectangular Channel

2009 ◽  
Author(s):  
Mei Wang ◽  
Yan Wen ◽  
Suizheng Qiu ◽  
Guanghui Su ◽  
Weifeng Ni
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Laminar Flow ◽  
Friction Factor ◽  
Water Flow ◽  
Single Phase ◽  
Rectangular Channel ◽  
Flow Region ◽  
Phase Water

The purpose of this study is to discover the differences of pressure drop and heat transfer of single-phase water flow between conventional channels and narrow rectangular channels. Furthermore, the differences between the level and the vertical channel have been studied. The gap of the test channel is 1.8mm. Compared with conventional channels, the narrow rectangular channel showed differences in both flow and heat transfer characteristics. The critical Reynolds number of transition from laminar flow to turbulent flow is 900∼1300, which is smaller compared with conventional channels. The friction factor is larger than that of the conventional channels and the correlation of friction factor with Reynolds number was given by experimental results. From the relation graph of Nusselt number and Reynolds number, the demarcation of the laminar flow region and turbulence flow region is obvious. In laminar region, Nusselt number almost remained constant and approximately consistent with numerical simulation results. While in turbulent region, Nusselt number increased significantly with increasing Reynolds number. A new Nusselt number correlation was obtained based on Dittus-Boelter equation, and the coefficients were less about 13% than that of Dittus-Boelter equation.

The Effect of Base Fluid Type in Nanofluids for Heat Transfer Enhancement in Microtubes

2016 ◽  
Vol 818 ◽  
pp. 12-22
Author(s):  
Bassam H. Salman ◽  
Hussein A. Mohammed ◽  
Akeel S. Kherbeet
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Ethylene Glycol ◽  
Base Fluid ◽  
Single Phase ◽  
Engine Oil ◽  
Convection Flow ◽  
Transfer Enhancement ◽  
Different Types ◽  
Forced Convection Flow

In this paper, single phase model was used to investigate the effect of base fluid in enhancing the heat transfer for forced convection flow of SiO2 in microtube. Four different types of base fluid such as water, ethylene glycol, engine oil and glycerin were used in this investigation. Reynolds number used was ranged from 10 to 120. The results are presented in terms of axial and wall temperature along the tube radius and tube axis, axial velocity and Nusselt number. The result shows that the glycerin has the highest Nusselt number followed by engine oil, ethylene glycol then water.

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