Flow and heat transfer enhancement in condensing water drops in steam flows

Gas turbine blade tips encounter large heat load as they are exposed to the high temperature gas. A common way to cool the blade and its tip is to design serpentine passages with 180-deg turns under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase the blade tip life time. This paper presents numerical predictions of turbulent fluid flow and heat transfer through two-pass channels with and without guide vanes placed in the turn regions using RANS turbulence modeling. The effects of adding guide vanes on the tip-wall heat transfer enhancement and the channel pressure loss were analyzed. The guide vanes have a height identical to that of the channel. The inlet Reynolds numbers are ranging from 100,000 to 600,000. The detailed three-dimensional fluid flow and heat transfer over the tip-walls are presented. The overall performances of several two-pass channels are also evaluated and compared. It is found that the tip heat transfer coefficients of the channels with guide vanes are 10∼60% higher than that of a channel without guide vanes, while the pressure loss might be reduced when the guide vanes are properly designed and located, otherwise the pressure loss is expected to be increased severely. It is suggested that the usage of proper guide vanes is a suitable way to augment the blade tip heat transfer and improve the flow structure, but is not the most effective way compared to the augmentation by surface modifications imposed on the tip-wall directly.

Download Full-text

Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini- and Microchannel Heat Exchangers for High Heat Flux Electronics Cooling

10.5772/intechopen.96015 ◽

2021 ◽

Author(s):

Jaime Rios ◽

Mehdi Kabirnajafi ◽

Takele Gameda ◽

Raid Mohammed ◽

Jiajun Xu

Keyword(s):

Heat Transfer ◽

Convective Heat Transfer ◽

Heat Transfer Enhancement ◽

Convective Heat ◽

Phase Flow ◽

Transfer Performance ◽

Transfer Enhancement ◽

Heat Transfer Characteristics ◽

Two Phase ◽

Flow And Heat Transfer

The present study experimentally and numerically investigates the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid, namely, ethanol/polyalphaolefin nanoemulsion, inside a conventionally manufactured minichannel of circular cross section and a microchannel heat exchanger of rectangular cross section manufactured additively using the Direct Metal Laser Sintering (DMLS) process. The experiments were conducted for single-phase flow of pure polyalphaolefin (PAO) and ethanol/PAO nanoemulsion fluids with two ethanol concentrations of 4 wt% and 8 wt% as well as for two-phase flow boiling of nanoemulsion fluids to study the effect of ethanol nanodroplets on the convective flow and heat transfer characteristics. Furthermore, the effects of flow regime of the working fluids on the heat transfer performance for both the minichannel and microchannel heat exchangers were examined within the laminar and transitional flow regimes. It was found that the ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of the pure PAO base fluid under both single- and two-phase flow regimes. While the concentration of nanoemulsion fluids did not reflect a remarkable distinction in single-phase heat transfer performance within the laminar regime, a significant heat transfer enhancement was observed using the nanoemulsion fluids upon entering the transitional flow regime. The heat transfer enhancement at higher concentrations of nanoemulsion within the transitional regime is mainly attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid. For two-phase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced when the ethanol nanodroplets underwent phase change. A comparative study on the flow and heat transfer characteristics was also implemented between the traditionally fabricated minichannel and additively manufactured microchannel of similar dimensions using the same working fluid of pure PAO and the same operating conditions. The results revealed that although the DMLS fabricated microchannel posed a higher pressure loss, a substantial heat transfer enhancement was achieved as compared to the minichannel heat exchanger tested under the same conditions. The non-post processed surface of the DMLS manufactured microchannel is likely to be the main contributor to the augmented heat transfer performance. Further studies are required to fully appreciate the possible mechanisms behind this phenomenon as well as the convective heat transfer properties of nanoemulsion fluids.

Download Full-text

Heat transfer enhancement in microchannels using ribs and secondary flows

International Journal of Numerical Methods for Heat &amp Fluid Flow ◽

10.1108/hff-04-2021-0253 ◽

2021 ◽

Vol ahead-of-print (ahead-of-print) ◽

Author(s):

Karthikeyan Paramanandam ◽

Venkatachalapathy S. ◽

Balamurugan Srinivasan

Keyword(s):

Heat Transfer ◽

Temperature Distribution ◽

Heat Transfer Enhancement ◽

Heat Transfer Rate ◽

Transfer Rate ◽

Secondary Flows ◽

Transfer Enhancement ◽

Heat Transfer Characteristics ◽

Content Type ◽

Flow And Heat Transfer

Purpose The purpose of this paper is to study the flow and heat transfer characteristics of microchannel heatsinks with ribs, cavities and secondary channels. The influence of length and width of the ribs on heat transfer enhancement, secondary flows, flow distribution and temperature distribution are examined at different Reynolds numbers. The effectiveness of each heatsink is evaluated using the performance factor. Design/methodology/approach A three-dimensional solid-fluid conjugate heat transfer numerical model is used to study the flow and heat transfer characteristics in microchannels. One symmetrical channel is adopted for the simulation to reduce the computational cost and time. Flow inside the channels is assumed to be single-phase and laminar. The governing equations are solved using finite volume method. Findings The numerical results are analyzed in terms of average Nusselt number ratio, average base temperature, friction factor ratio, pressure variation inside the channel, temperature distribution, velocity distribution inside the channel, mass flow rate distribution inside the secondary channels and performance factor of each microchannels. Results indicate that impact of rib width is higher in enhancing the heat transfer when compared with its length but with a penalty on the pressure drop. The combined effects of secondary channels, ribs and cavities helps to lower the temperature of the microchannel heat sink and enhances the heat transfer rate. Practical implications The fabrication of microchannels are complex, but recent advancements in the additive manufacturing techniques makes the fabrication of the design considered in this numerical study feasible. Originality/value The proposed microchannel heatsink can be used in practical applications to reduce the thermal resistance, and it augments the heat transfer rate when compared with the baseline design.

Download Full-text

Analysis of Flow and Heat Transfer Enhancement in a Horizontal Pipe with PTT and V-cut Twisted Tape

Strad Research ◽

10.37896/sr7.9/067 ◽

2020 ◽

Vol 7 (9) ◽

Keyword(s):

Heat Transfer ◽

Heat Transfer Enhancement ◽

Twisted Tape ◽

Transfer Enhancement ◽

Horizontal Pipe ◽

Flow And Heat Transfer

Download Full-text

Flow and Heat Transfer Characteristics in Channels With Groove–Protrusions and Combination Effect With Ribs

Journal of Heat Transfer ◽

10.1115/1.4031077 ◽

2015 ◽

Vol 138 (1) ◽

Cited By ~ 1

Author(s):

Lu Zheng ◽

Yonghui Xie ◽

Di Zhang ◽

Haoning Shi

Keyword(s):

Heat Transfer ◽

Heat Transfer Enhancement ◽

Rectangular Channel ◽

Leading Edge ◽

Small Scale ◽

Combination Effect ◽

Transfer Enhancement ◽

Heat Transfer Characteristics ◽

Transfer Characteristics ◽

Flow And Heat Transfer

Passive flow control and heat transfer enhancement technique has become an attractive method for device internal cooling with low resistance penalty. In the present paper, the flow and heat transfer characteristics in the small scale rectangular channel with different groove–protrusions are investigated numerically. Furthermore, the combination effect with ribs is studied. The numerical results show that on the groove side, the flow separation mainly occurs at the leading edge, and the reattachment mainly occurs at the trailing edge in accordance with the local Nusselt number distribution. On the protrusion side, the separation mainly occurs at the protrusion back porch and enhances the heat transfer at the leading edge of the downstream adjacent groove. The rectangle case provides the highest dimensionless heat transfer enhancement coefficient Nu/Nu0, dimensionless resistance coefficient f/f0, and thermal performance (TP) with the highest sensitivity of Re. When ribs are employed, the separation bubble sizes prominently decrease, especially inside the second and third grooves. The Nu/Nu0 values significantly increase when ribs are arranged, and the one-row case provides the highest heat transfer enhancement by ribs. Besides, the two-row case provides the highest Nu/Nu0 value without ribs, and the three-row case shows the lowest Nu/Nu0 value whether ribs are arranged or not.

Download Full-text

Laminar Flow and Heat Transfer Characteristics of Colloid Suspensions in Water: A Numerical Study

Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer ◽

10.1115/ht2009-88214 ◽

2009 ◽

Author(s):

Khalid N. Alammar ◽

Lin-wen Hu

Keyword(s):

Heat Transfer ◽

Laminar Flow ◽

Heat Transfer Enhancement ◽

Volume Fraction ◽

Numerical Study ◽

Volume Flow Rate ◽

Transfer Enhancement ◽

Heat Transfer Characteristics ◽

Transfer Characteristics ◽

Flow And Heat Transfer

Numerical analysis is performed to examine axisymmetric laminar flow and heat transfer characteristics of colloidal dispersions of nanoparticles in water (nanofluids). Effect of volume fraction on flow and heat transfer characteristics is investigated. Four different materials, Alumina, Copper, Copper Oxide, and Graphite are considered. Heat transfer and property measurements were conducted previously for Alumina nanofluid. The measurements have shown that nanofluids can behave as homogeneous mixtures. It is found that oxide-based nanofluids offer the least heat transfer enhancement compared to elements-based nanofluids. When normalized by friction pressure drop, it is shown that graphite can have the highest effective heat transfer enhancement. For a given volume flow rate, all nanofluids exhibited linear increase in heat transfer enhancement with increasing colloids volume fraction, up to 0.05.

Download Full-text

Numerical Study of the Effects of Different Buoyancy Models on Supercritical Flow and Heat Transfer

Volume 4: Heat and Mass Transfer Under Extreme Conditions; Environmental Heat Transfer; Computational Heat Transfer; Visualization of Heat Transfer; Heat Transfer Education and Future Directions in Heat Transfer; Nuclear Energy ◽

10.1115/ht2013-17295 ◽

2013 ◽

Cited By ~ 1

Author(s):

X. Y. Xu ◽

T. Ma ◽

M. Zeng ◽

Q. W. Wang

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Heat Transfer Enhancement ◽

Wall Temperature ◽

Mass Flux ◽

Wall Heat Flux ◽

Turbulent Heat ◽

Transfer Enhancement ◽

Temperature Prediction ◽

Flow And Heat Transfer

Due to the dramatic changes in physical properties, the flow and heat transfer in supercritical fluid are significantly affected by buoyancy effects, especially when the ratio of inlet mass flux and wall heat flux is relatively small. In this study, the heat transfer of supercritical water in uniformly heated vertical tube is numerically investigated with different buoyancy models which are based on different calculation methods of the turbulent heat flux. The applicabilities of these buoyancy models are analyzed both in heat transfer enhancement and deterioration conditions. The simulation results show that these buoyancy models make few differences and give good wall temperature prediction in heat transfer enhancement condition when the ratio of inlet mass flux and wall heat flux is very small. With the increase of wall heat flux, the accuracy of wall temperature prediction reduces, and the differences between these buoyancy models become larger. No buoyancy model can currently make accurate wall temperature prediction in deterioration condition in this study.

Download Full-text

CFD Simulations of Flow and Heat Transfer in a Zigzag Channel With Flow Pulsation

Volume 7: Fluid Flow, Heat Transfer and Thermal Systems, Parts A and B ◽

10.1115/imece2010-38596 ◽

2010 ◽

Author(s):

Bolaji O. Olayiwola ◽

Gerhard Schaldach ◽

Peter Walzel

Keyword(s):

Heat Transfer ◽

Reynolds Number ◽

Heat Transfer Enhancement ◽

Steady Flow ◽

Inclination Angle ◽

Oscillation Amplitude ◽

Fluid Mixing ◽

Transfer Enhancement ◽

Flow And Heat Transfer ◽

Flow Through

Heat transfer enhancement by pulsating flow in a zigzag channel has been numerically studied using a commercial CFD software for the ranges of laminar flow 0 < Re < 550. The influence of inclination angle α of the zigzag channel and oscillation parameters is investigated. The amplitude of the pulsatile flow was varied between 0.5 mm and 4 mm. The frequency f ranges between 0.5 Hz and 5.5 Hz. For steady flow, fluid mixing is promoted by self induced fluctuation due to the instability of the flow. The Reynolds number Re for the occurrence of significant eddy decreases with increase of the inclination angle of the channel. Superposition of oscillation additionally promotes further fluid mixing by the propagation of different scales of vortices. In comparison to straight channels, significant heat transfer in the laminar regime is possible using a zigzag channel with inclination angle greater than 15°. Further intensification of the heat transfer is possible with superposition of oscillation on the main flow through the channel. However, the heat transfer enhancement due to imposed oscillation is found to increase with decreasing Reynolds number. The effect of the imposed oscillation yields heat transfer enhancement E of up to 1.41 when compared with steady flow in zigzag channel at Reynolds number Re = 107, frequency f = 2.17 Hz and oscillation amplitude A = 1mm using a zigzag channel with an inclination angle α = 15°. Further heat transfer enhancement E of up to 1.80 at the same flow and oscillation conditions is possible with a zigzag channel having inclination angle α = 45°. The influence of oscillation frequency on the heat transfer enhancement E becomes significant as soon as the Womersley number W > 41.32. The effect of superposition of oscillation is not significant using a zigzag channel with inclination angle α = 60°. When the oscillation amplitude is increased up to 4 mm at Reynolds number Re = 107, frequency f = 2.17 Hz and inclination angle α = 45°, the heat transfer enhancement E of about 3.3 is obtained.

Download Full-text