Average Heat Transfer Coefficient Ratio Research Articles

CFD Analysis for the Variation of Velocity inside Different Types of Ribs

Abstract: Heat exchanger is used to accomplish the industrial demand. Heat Exchanger are necessary for nuclear reactors and in chemical engineering, for of large quantity of heat is conveying in a slight space with high heat transmission rates and slight habitation time distributions even it suffers through a disadvantage of larger pressure drop. From my analysis, it is found that the uses of ribs inside the solar heat exchanger increase the heat transfer with increase in Reynolds number. Through analysis, it is also found that the value of heat transfer coefficient increases with increase in Reynolds number, heat transfer coefficient is maximum for rectangular shape rib in both the cases that are during using water and nano fluid as a working fluid. The average heat transfer coefficient ratio graph shows that rib used in the solar heat exchanger with nano fluid flow shows more heat transfer enhancement as compared to a solar heat exchanger with simple water flow. Overall it is found that rectangular shape ribs show more heat transfer coefficient as compared to other rib.

Numerical study on forced convection of water-based suspensions of nanoencapsulated PCM particles/Al2O3 nanoparticles in a mini-channel heat sink

The contributions of Al2O3 nanoparticles and nanoencapsulated PCM particles for enhancing the heat transfer in the rectangular mini-channel heat sink are compared in this work. A three-dimensional numerical method is used to simulate the forced convection heat dissipation of water-based suspensions of nanoencapsulated PCM particles/Al2O3 nanoparticles in the mini-channel heat sink. A homogenous mixture formulation by means of the pseudo-vorticity-velocity formulation together with the energy equation is adopted to evaluate the cooling characteristics. Several important factors in the system such as the friction factor, the thermal resistance, the figure of merit, and the average heat transfer coefficient ratio are investigated at various concentrations of Al2O3 nanoparticles and nanoencapsulated PCM particles, flow Reynolds numbers, and heat fluxes. The numerical results clearly reveal that the maximum thermal resistance reduction of 1.2% and 6.9% can be observed by adding the nanoencapsulated PCM suspension with the concentrations of 2% and 10%, respectively. For the case of water-based suspension of Al2O3 nanoparticles, the heat transfer coefficient ratios are larger than unity for all values of heat flux and Reynolds number. The usage of Al2O3 nanoparticles provides better cooling performance as compared with the case of nanoencapsulated PCM particles. As a result, the beneficial effect of Al2O3 nanoparticles is much greater as compared with that of nanoencapsulated PCM particles.

Toward improved heat dissipation of the turbulent regime over backward-facing step for the AL2O3-water nanofluids: An experimental approach

Experimental study of nanofluid flow and heat transfer to fully developed turbulent forced convection flow in a uniformly heated tubular horizontal backward-facing step has reported in the present study. To study the forced convective heat transfer coefficient in the turbulent regime, an experimental study is performed at a different weight concentration of Al2O3 nanoparticles. The experiment had conducted for water and Al2O3 -water nanofluid for the concentration range of 0 to 0.1 wt.% and Reynolds number of 4000 to 16000. The average heat transfer coefficient ratio increases significantly as Reynolds number increasing, increased from 9.6% at Reynolds number of 4000 to 26.3% at Reynolds number of 16000 at the constant weight concentration of 0.1%. The Al2O3 water nanofluid exhibited excellent thermal performance in the tube with a backwardfacing step in comparison to distilled water. However, the pressure losses increased with the increase of the Reynolds number and/or the weight concentrations, but the enhancement rates were insignificant.

Functionalization and exfoliation of graphite into mono layer graphene for improved heat dissipation

A novel synthesis procedure is presented for preparing single layer graphene (SGr) with high specific surface area (SSA). The SGr has been introduced as a promising additive for preparing super conductive nanofluids for high performance heat transfer applications. Owing to high SSA and unique degree of functionalization with hydrophilic groups, the SGr demonstrates an excellent colloidal stability and thermo-physical properties as an additive in aqueous media. Using UV–vis spectrometer, a highest dispersion stability of 0.866-relative concentration was reached after 30 days from preparation. Water-based SGr nanofluids illustrated fairly Newtonian behavior. Nanofluid containing SGr sheets with SSA of 761 m2/g and weight concentration of 0.01% showed the largest increase in thermal conductivity, i.e., from 0.748 to 0.795 W/m K as the temperature increased from 20 to 50 °C. The average heat transfer coefficient ratio (hnf/hbf)ave showed a downward trend with increasing of Reynolds number, decreasing from 233.5% at Re of 2000 to only 83.8% at Re of 16,000 at the constant weight concentration of 0.01%. Finally, the water-based SGR nanofluids were shown to be dramatically more effective in the duct with a backward-facing step in terms of overall thermal performance e.g., performance index and pumping power, and rheological properties including effective viscosity in comparison to distilled water.

A NUMERICAL STUDY OF NANOFLUID FORCED CONVECTION IN A POROUS CHANNEL WITH DISCRETE HEAT SOURCES

A numerical simulation is carried out to investigate the laminar forced convection flow of Al2O3−water nanofluid in a porous parallel plates channel with discrete heat sources using a single-phase approach. The heat sources are placed on the bottom wall of the channel, and the remaining surfaces of the channel are considered adiabatic. The effects of Darcy number (Da = 10−1, 10−2, 10−4, and 10−6), particle volume fraction (φ = 0% (distilled water), 3%, 5%, and 9%), heat flux of heat sources (q = 5000,10,000, 20,000, and 30,000 W/m2), porosity (0.4, 0.6, 0.8, and 0.95), and thermal conductivity ratio (ks/kf = 1, 5,10, and 30) on the average heat transfer coefficient (h), pressure drop (ΔP), thermal-hydraulic performance (η), and wall temperature (Tw) are investigated. A remarkable decrease in wall temperature is observed, especially on the heat sources. It is founded that heat transfer and pressure drop increase for all cases as volume fraction increases. Results also reveal that in a fixed volume fraction, the average heat transfer coefficient ratio, referring to the values calculated for base fluid, increases as heat flux and porosity increase or thermal conductivity ratio decreases. No change in the average heat transfer coefficient ratio is observed with Darcy number variation. Furthermore, it is found that the pressure ratio, referring to the values calculated for base fluid, is approximately independent of all considered parameters, except volume fraction.

Forced Convective Boiling Heat Transfer Characteristics and Critical Heat Flux in Helically Coiled Tubes.

Heat transfer and CHF (Critical heat flux) characteristics of flow boiling of R-113 in helically coiled tubes were experimentally investigated. Two coiled tubes with coil diameters of 0.165 and 0.32m, and 10mm I.D. were tested at a pressure of 0.39 MPa. In the nucleate boiling region, circumferential difference in heat transfer was not clarified qualitatively as well as quantitatively. The ratio of circumferential average boiling heat transfer coefficient to that of the single-phase flow in a curved tube, hTP/hLO, was found to be rather close to the correlations of Pujol-Stenning and Ueda-Kim for a straight tube flow. In the high quality region, the heat transfer coefficient was highest at the coil outside and lowest at the inside, and the average heat transfer coefficient ratio hTP/hLo was expressed in terms of Martinelli parameter, Xtt, only. A correlation applicable to both the nucleate boiling and forced convective evaporation regions was proposed. The CHF of the coiled tube was higher than that of the straight tube when the coil diameter was small, but it became considerably low at low mass velocity when the coil diameter was large.

Heat Transfer of Air/Water Two-Phase Flow in Helicoidal Pipes

Heat transfer of air/water two-phase flow in helicoidal pipes is experimentally investigated in this study. Three test sections were tested in axially horizontal, vertical, and inclined orientations. It has been found that the ratio of the average heat transfer coefficient to that of the water flow is affected by the water flow rate along with the Lockhart–Martinelli parameter. For a fixed water flow rate, there is a maximum heat transfer coefficient as the air fow rate increases. The results indicate that tube diameter has a significant effect on the average heat transfer coefficient ratio. For large-tube-diameter coils, average heat transfer results vary for different orientations. For small-tube-diameter coils, orientation has an insignificant effect on the average heat transfer coefficient. Based on the experimental data, a set of correlations is proposed for the average heat transfer coefficient ratio versus the liquid superficial Reynolds number and Lockhart–Martinelli parameter.