scholarly journals Determination of Limiting Heat Flux for The Inception of Nucleate Boiling Regime for Crude Oils

2021 ◽  
Vol 85 (2) ◽  
pp. 115-127
Author(s):  
Obaid ur Rehman ◽  
Marappa Gounder Ramasamy ◽  
Nor Erniza M Rozali ◽  
Umesh B. Deshannavar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Nucleate Boiling ◽  
Crude Oils ◽  
Bulk Temperature ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Accuracy Of Results

Finding the limiting heat flux above which nucleate boiling starts and below which forced convective heat transfer exists is a crucial task for the accuracy of results from crude oil fouling tests. In this study, crude oils from two sources were tested at bulk temperatures of 100, 120 and 140 °C and different velocities. Heat transfer coefficient increased gradually with bulk temperature indicated lowering of the viscosity at high temperatures which promoted turbulence and enhanced heat transfer. The velocity effects were similar to that of bulk temperatures on maximum heat transfer coefficient while less heat flux was required to achieve the same surface temperature at lower velocities. Deshannavar and Ramasamy’s model to predict maximum heat flux was compared with experimental results and a poor estimation was observed for the crude oils tested.

Performance of a Flexible Evaporator for Loop Heat Pipe Applications

2008 ◽  
Author(s):  
Mukta S. Limaye ◽  
James F. Klausner
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Heat Fluxes ◽  
Loop Heat Pipe ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Maximum Heat Flux

A flat and flexible evaporator, which conforms to contoured surfaces, has been developed for loop heat pipe applications. A loop heat pipe (LHP) is a passive, two phase heat transfer device that uses a porous membrane in the evaporator to circulate fluid. A number of flexible membranes have been tested as evaporator wicks that have a length of 12.7 cm and heated area of 50.6 cm2. For cellulose, polyethylene, and blotting paper membranes, maximum heat fluxes of 0.43, 1.5 and 2.9 W/cm2 have been observed, respectively. The maximum heat transfer coefficients measured for these membranes are 551, 876, and 2100 W/m2-K, respectively. The best performance was observed by a membrane made of a fibrous cotton matrix, typically used as gauze. This material has a large pore size and high wettability with water. When tested in a rigid, brass evaporator, the maximum heat flux observed is 5.95 W/cm2, and the maximum heat transfer coefficient is 2865 W/m2-K. A flexible evaporator is fabricated using a heat sealable, flexible barrier pouch, and the cotton matrix membrane is sealed inside. The maximum measured heat flux for the flexible evaporator is 3.2 W/cm2 and maximum measured heat transfer coefficient is 1165 W/m2-K. The observed reduction in heat transfer as compared to the rigid evaporator is due to the poor contact between the evaporator and membrane. It is concluded that for the flexible evaporator membranes considered, the heat transfer mechanism is boiling and the maximum heat flux is limited by the wicking rate of the membrane. For a given membrane, the wicking rate increases with a reduction in the wicking length and decreases with an increasing rate of evaporation. To further improve the performance of the flexible evaporator, it is necessary to ensure efficient vapor removal from the evaporator as well as maintaining good contact between the membrane and the evaporator surface.

Experimental Investigation on Pulsating Horizontal Heating of a Water-Filled Enclosure

1996 ◽  
Vol 118 (4) ◽  
pp. 889-896 ◽  
Author(s):  
B. V. Antohe ◽  
J. L. Lage
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Experimental Results ◽  
Periodic Regime ◽  
Heating Period ◽  
Heating Wall ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Experimental results of the natural convection generated by the time periodic horizontal heating of a square cross-section enclosure filled with water are reported. A pulsating (on/off) heat flux is delivered to the heating wall of the enclosure, with the opposite wall cooled by a high thermal capacitance system. All other surfaces are insulated. Heating periods from 32 to 1600 seconds and cycle-averaged heat-flux based Rayleigh numbers from 2.5 × 108 to 1.0 × 109 are considered. Results presented in terms of time series, phase-plane portraits, and cyclic evolution of surface-averaged cooling and heating wall temperatures illustrate the main characteristics of the evolution to periodic regime. Also presented are the cycle-averaged heat transfer coefficient versus heating period, and the corresponding average Nusselt number versus Rayleigh number for various heating frequencies. These results, which support published theoretical and numerical analysis, indicate that by tuning the heating period properly, the heat transfer across an enclosure can be enhanced. The results also reveal that short heating periods hinder the convection within the enclosure, in general (e.g., for Ra = 7.5 × 108 and a period of 32 s the heat transfer coefficient is 13 percent smaller than the steady heating value). The sensitivity of the transport phenomenon to pulsating heat is shown to depend strongly on Ra. Finally, a correlation for estimating the maximum heat transfer coefficient, derived from the experimental results, is presented.

Saturation Nucleate Boiling and Correlations for PF-5060 Dielectric Liquid on Inclined Rough Copper Surfaces

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
Mohamed S. El-Genk ◽  
Arthur Suszko
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Nucleate Boiling ◽  
Dielectric Liquid ◽  
Average Roughness ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Inclination Angles ◽  
Boiling Heat Transfer Coefficient

Saturation pool boiling experiments of degassed PF-5060 dielectric liquid investigated nucleate boiling on 13 Cu surfaces with average roughness, Ra, of 0.039 (smooth polished) to 1.79 μm at six inclination angles, θ, from 0 deg (upward facing) to 180 deg (downward facing). Values of the nucleate boiling heat transfer coefficient, hNB, in the upward facing orientation increase with increasing surface roughness and are correlated in terms of the applied heat flux, q: hNB = A qB. The exponent “B” decreases from 0.81 to 0.69 as Ra increases from 0.039 to 1.79 μm, while the coefficient “A” increases with Ra to the power 0.24. The values of the maximum heat transfer coefficient, hMNB, which occurs near the end of the fully developed nucleate boiling region, increase with increasing Ra and decreasing inclination angle. In the upward facing orientation, hNB increases by ∼58% with increasing Ra from 0.134 to 1.79 μm, while hMNB increases by more than 150% compared with that on smooth-polished Cu. Values of hMNB in the downward facing orientation are ∼40% of those in the upward facing orientation.

scholarly journals Enhancement of Nucleate Pool Boiling Heat Transfer with Sodium Oleate

2017 ◽  
Vol 29 (1) ◽  
pp. 44-48
Author(s):  
KM Tanvir Ahmmed ◽  
Sultana Razia Syeda
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Sodium Oleate ◽  
Surfactant Concentration ◽  
Chemical Engineering ◽  
Pool Boiling ◽  
Nucleate Pool Boiling ◽  
Maximum Heat ◽  
Maximum Heat Transfer

In this study saturated nucleate pool boiling of water with sodium oleate surfactant on a horizontal cylindrical heater surface has been investigated experimentally and compared with that of demineralized water. The concentration of sodium oleate in water was 100-300 ppm. The experimental results show that a small amount of surfactant enhances the heat transfer coefficient significantly. At low surfactant concentrations, heat transfer coefficient increases with increasing surfactant concentration in water. The maximum heat transfer enhancement is found to be at 250 ppm of sodium oleate solution. By adding more surfactant to water, heat transfer coefficient is found to be lowered. Surface tension of different concentration of sodium oleate solutions is measured. It is observed that the maximum heat transfer coefficient is obtained at a surfactant concentration that corresponds to the critical micelle concentration (cmc) of the sodium oleate solution.Journal of Chemical Engineering, Vol. 29, No. 1, 2017: 44-48

scholarly journals Comparison of heat transfer performance of ZnO-PG, α-Al2O3-PG, and γ-Al2O3-PG nanofluids in car radiator

2019 ◽  
Vol 9 ◽  
pp. 184798041987646 ◽  
Author(s):  
XiaoRong Zhou ◽  
Yi Wang ◽  
Kai Zheng ◽  
Haozhong Huang
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Propylene Glycol ◽  
Flow Rate ◽  
Volume Concentration ◽  
Maximum Heat ◽  
Cooling Performance ◽  
Maximum Heat Transfer ◽  
The Heat Transfer Coefficient

In this study, the cooling performance of nanofluids in car radiators was investigated. A car radiator, temperature measuring instrument, and other components were used to set up the experimental device, and the temperature of nanofluids passing through the radiator was measured by this device. Three kinds of nanoparticles, γ-Al2O3, α-Al2O3, and ZnO, were added to propylene glycol to prepared nanofluids, and the effects of nanoparticle size and type, volume concentration, initial temperature, and flow rate were tested. The results indicated that the heat transfer coefficients of all nanofluids first increased and then decreased with an increase in volume concentration. The ZnO-propylene glycol nanofluid reached a maximum heat transfer coefficient at 0.3 vol%, and the coefficient decreased by 25.6% with an increase in volume concentration from 0.3 vol% to 0.5 vol%. Smaller particles provided a better cooling performance, and the 0.1 vol% γ-Al2O3-propylene glycol nanofluid had a 19.9% increase in heat transfer coefficient compared with that of α-Al2O3-propylene glycol. An increase in flow rate resulted in a 10.5% increase in the heat transfer coefficient of the 0.5 vol% α-Al2O3-propylene glycol nanofluid. In addition, the experimental temperature range of 40–60°C improved the heat transfer coefficient of the 0.2 vol% ZnO-propylene glycol nanofluid by 46.4%.

scholarly journals Deterioration in Heat Transfer to Fluids at Supercritical Pressure and High Heat Fluxes

1969 ◽  
Vol 91 (1) ◽  
pp. 27-36 ◽  
Author(s):  
B. S. Shiralkar ◽  
Peter Griffith
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
High Heat ◽  
Supercritical Pressure ◽  
Heat Fluxes ◽  
Bulk Temperature ◽  
Operating Pressure ◽  
The Heat Transfer Coefficient

At slightly supercritical pressure and in the neighborhood of the pseudocritical temperature (which corresponds to the peak in the specific heat at the operating pressure), the heat transfer coefficient between fluid and tube wall is strongly dependent on the heat flux. For large heat fluxes, a marked deterioration takes place in the heat transfer coefficient in the region where the bulk temperature is below the pseudocritical temperature and the wall temperature above the pseudocritical temperature. Equations have been developed to predict the deterioration in heat transfer at high heat fluxes and the results compared with previously available results for steam. Experiments have been performed with carbon dioxide for additional comparison. Limits of safe operation for a supercritical pressure heat exchanger in terms of the allowable heat flux for a particular flow rate have been determined theoretically and experimentally.

Effect of Liquid Properties on Phase-Change Heat Transfer in Porous Wick Structures

2015 ◽  
Vol 138 (3) ◽  
Author(s):  
Steve Q. Cai ◽  
Avijit Bhunia
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Phase Change ◽  
Transfer Coefficient ◽  
Mode Transition ◽  
Maximum Heat ◽  
Maximum Heat Flux ◽  
Phase Change Heat Transfer ◽  
The Heat Transfer Coefficient

In a heat pipe, operating fluid saturates wick structures system and establishes a capillary-driven circulation loop for heat transfer. Thus, the thermophysical properties of the operating fluid inevitably impact the transitions of phase-change mode and the capability of heat transfer, which determine both the design and development of the associated heat pipe systems. This article investigates the effect of liquid properties on phase-change heat transfer. Two different copper wick structures, cubic and cylindrical in cross section, 340 μm in height and 150 μm in diameter or width, are fabricated using an electroplating technique. The phase-change phenomena inside these wick structures are observed at various heat fluxes. The corresponding heat transfer characteristics are measured for three different working liquids: water, ethanol, and Novec 7200. Three distinct modes of the phase-change process are identified: (1) evaporation on liquid–vapor interface, (2) nucleate boiling with interfacial evaporation, and (3) boiling enhanced interface evaporation. Transitions between the three modes depend on heat flux and liquid properties. In addition to the mode transition, liquid properties also dictate the maximum heat flux and the heat transfer coefficient. A quantitative characterization shows that the maximum heat flux scales with Merit number, a dimensionless number connecting liquid density, surface tension, latent heat of vaporization, and viscosity. The heat transfer coefficient, on the other hand, is dictated by the thermal conductivity of the liquid. A complex interaction between the mode transition and liquid properties is reflected in Novec 7200. In spite of having the lowest thermal conductivity among the three liquids, an early transition to the mode of the boiling enhanced interface evaporation leads to a higher heat transfer coefficient at low heat flux.

scholarly journals Thermal performance prediction of heat pipe with TiO2 nanofluids using RSM

2021 ◽  
pp. 199-199
Author(s):  
Lakshmi Reddy ◽  
Srinivasa Bayyapureddy Reddy ◽  
Kakumani Govindarajulu
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Thermal Performance ◽  
Tilt Angle ◽  
Heat Load ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Heat pipe is a two phase heat transfer device with high effective thermal conductivity and transfer huge amount of heat with minimum temperature gradient in between evaporator and condenser section. This paper objective is to predict the thermal performance in terms of thermal resistance (R) and heat transfer coefficient (h) of screen mesh wick heat pipe with DI water-TiO2 as working fluid. The input process parameters of heat pipe such as heat load (Q), tilt angle (?) and concentration of nanofluid (?) were modeled and optimized by utilizing Response Surface Methodology (RSM) with MiniTab-17 software to attain minimum thermal resistance and maximum heat transfer coefficient. The minimum thermal resistance of 0.1764 0C/W and maximum heat transfer coefficient of 1411.52 W/m2 0C was obtained under the optimized conditions of 200 W heat load, 57.20 tilt angle and 0.159 vol. % concentration of nano-fluid.

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