Measurements of mass transfer between particles and gas in packed tubes at very low tube to particle diameter ratios

Experimental results for the mass transfer between spherical naphthalene particles and air in packed tubes of very low tube to particle diameter ratio (D/d=1.41, 1.98, and 3.77) are presented. During the experiments the Reynolds number (2.4Re ₀<1500), the=" bed=" length,=" and=" the=" test=" temperature=" have=" been=" varied.=" data=" reduction=" has=" been=" carried=" out=" with=" and=" without=" accounting=" for=" axial=" dispersion=" in=" the=" model.=" the=" measured=" sherwood=" numbers=" are=" compared=" with=" the=" predictions=" according=" to=" the=" correlation=" of=" gnielinski=" and=" of=" wakao/funazkri,=" originally=" developed=" for=" large=" packed=" beds.=" the=" porosity=" of=" packings=" at=">D/d-ratios is discussed.Es werden Versuchsergebnisse über die Stoffübertragung zwischen kugelförmigen Naphthalinpartikeln und Luft in Festbetten mit sehr kleinem Verhältnis zwischen dem Rohr- und dem Partikeldurchmesser (D/d=1,41, 1,98 und 3,77) mitgeteilt. Während der Experimente wurden die Reynoldszahl (2,4Re ₀<1500), die=" bettlänge=" und=" die=" temperatur=" variiert.=" die=" versuchsauswertung=" erfolgte=" sowohl=" mit=" als=" auch=" ohne=" berücksichtigung=" der=" axialen=" dispersion.=" die=" experimentell=" ermittelten=" sherwoodzahlen=" werden=" mit=" den=" voraussagen=" der=" korrelationen=" von=" gnielinski=" und=" von=" wakao/funazkri=" verglichen;=" beide=" korrelationen=" wurden=" für=" betten=" mit=" einem=">D/d-Verhältnis entwickelt. Außerdem wird die Porosität von Betten im Bereich kleinerD/d-Quotienten behandelt.Dedicated to Prof. Dr.-Ing. K. Stephan on the occasion of his 60th birthday     

Analytical study of critical heat flux in vertical tubes with small length to diameter ratios

An analytical study is made of the critical axial heat flux in heated closed end vertical tubes in the case of small length to diameter ratios. An expression for the critical heat flux is obtained and its dependency on the dimensionless diameter is shown. Results are compared with the previous analysis for large length to diameter ratios and the available experimental data on vertical closed end tubes, vertical thermosyphons and wickless heat pipes. A high degree of consistancy between the present analysis and these experiments is obtained.     

Heat and mass transfer characteristics of a horizontal tube falling film absorber with small diameter tubes

This paper presents experimental results of the heat and mass transfer characteristics of a water–LiBr horizontal tube absorber made of small diameter tubes. The experimental set up includes a tube absorber, a generator, solution distribution system and cooling water system. Three different tube diameters of 15.88, 12.70 and 9.52 mm have been installed inside the absorber to investigate the effect of the tube diameter on the absorber performance. The experimental results show that the heat and mass transfer performance of the absorber increases as the tube diameter decreases. A comparison of the heat and mass transfer coefficients of the present study agree reasonable well with that of the previous studies.     

A new numerical method for determining heat transfer and packing distribution in particle heat exchangers for concentrated solar power

The complex nature of the physics of solid-gas interactions in concentrated solar particle heat exchangers signifies the need to develop new and cutting-edge numerical models to understand these interactions with the overarching goal of optimizing industrial solar thermal processes. To this end, a coupled computational fluid dynamics and discrete element method is developed to unravel near-wall particle flow physics of solar industrial heat exchangers. In addition, advanced post-processing functions are developed to provide a high-end data visualization and quantitative assessment of the packing distribution of solar particle heat exchangers. The validated numerical model shows that the particle temperature varies considerably throughout the entire fluid filled packed particle bed and it is shown that thermal radiation contribution becomes more profound at higher operating temperatures, namely 1073–1173 K. Also, the temperatures and solid volume fractions of the near-wall particles differ greatly compared to the bulk particles. The methods presented herein can be implemented by engineers and scientists to evaluate near-wall packing distributions and thermal characteristics, which would be useful for optimizing the geometric morphology of solar industrial heat exchangers.     

Effect of nanoparticles mean diameter on mixed convection heat transfer of a nanofluid in a horizontal tube

Fully developed mixed convection of a nanofluid (water/Al₂O₃) has been studied numerically. Two-phase mixture model has been used to investigate the effects of nanoparticles mean diameter on the flow parameters. The calculated results demonstrate that the convection heat transfer coefficient significantly increases with decreasing the nanoparticles means diameter. However it does not significantly change the hydrodynamics parameters. Nanoparticles distribution at the tube cross section shows that the non-uniformity of the particles distribution augments when using larger nanoparticles and/or considering relatively high value of the Grashof numbers.     

Boiling heat transfer in a vertical tube with freon 114

Test results for boiling heat transfer coefficients for R114 in vertical stainless tubes are reported both for upflow and downflow direction. Results are compared with different formulas given in the literature. A Recommendation of the numerical value ofC _sf in Rohsenow's formula for fully developed nucleation boiling for R114/stainless steel combination is given.     

Aspects of coiled tube heat transfer

Enhancement of heat transfer performance, beyond that normally achieved in curved tube flows, is demonstrated for pulsatile flows and for developing flows. In the former, increases of greater than 20% are obtained and in the latter, a maximum increase of 60% is obtained.

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Wärmeübergang in gewendelten Rohren

Zusammenfassung Es wird gezeigt, daß der Wärmeaustausch in gebogenen Rohren gegenüber herkömmlichen Strömungen vergrößert werden kann. Für pulsierende Strömungen werden Steigungen von mehr als 20%, für Einlaufströmungen von maximal 60% beobachtet.

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Nomenclature a Pipe radius - De Dean number= - K Pressure amplitude ratio - Nu Nusselt number - Pr Prandtl number - R Coil radius - Re Reynolds number - T Dimensionless temperature - Z Dimensionless axial length - Dimensionless frequency= - _m Mean boundary layer thickness - Excitation frequency - Kinematic viscosity - Dimensionless wave-length Dedicated to Professor E. R. G. Eckert on the occasion of his 80th birthday     

Evaporation flow pattern and heat transfer of R-22 and R-134a in small diameter tubes

The flow patterns and heat transfer coefficients of R-22 and R-134a during evaporation in small diameter tubes were investigated experimentally. The evaporation flow patterns of R-22 and R-134a were observed in Pyrex sight glass tubes with 2 and 8 mm diameter tube, and heat transfer coefficients were measured in smooth and horizontal copper tubes with 1.77, 3.36 and 5.35 mm diameter tube, respectively. In the flow patterns during evaporation process, the annular flows in 2 mm glass tube occurred at a relatively lower vapor quality compared to 8 mm glass tube. The flow patterns in 2 mm glass tube did not agree with the Mandhane’s flow pattern maps. The evaporation heat transfer coefficients in the small diameter tubes (d _i  < 6 mm) were observed to be strongly affected by tube diameters, and to differ from those in the large diameter tubes. The heat transfer coefficients of 1.77 mm tube were higher than those of 3.36 mm and 5.35 mm tube. Most of the existing correlations failed to predict the evaporation heat transfer coefficient in small diameter tubes. Therefore, based on the experimental data, the new correlation is proposed to predict the evaporation heat transfer coefficients of R-22 and R-134a in small diameter tubes.     

Calculation of overall heat transfer coefficients in a triple tube heat exchanger

In previous studies, calculation of overall heat transfer coefficients in a triple tube heat exchanger (TTHE) involved assumptions or approaches those are not valid in all cases. In this study a more generic way of calculating overall heat transfer coefficients in a TTHE has been developed. Consequently, temperature profiles of all streams in a TTHE in the axial direction were determined. An effective overall heat transfer coefficient that is related to the total resistance to heat transfer in the TTHE, was also determined to facilitate comparison of a TTHE to an equivalent double tube heat exchanger.     

Forced convective flow drag and heat transfer characteristics of carbon nanotube suspensions in a horizontal small tube

This study paid attention to the effect of fluid temperatures on the forced convective flow drag and heat transfer characteristics of multi-wall carbon nanotube (MWNTs)-water suspensions without any surfactants. The experiments were carried out under the two fixed average fluid temperatures of 29 and 58°C. A horizontal small stainless steel tube with an inner diameter of 1.02 mm was used as the test section. The experiment results show that the flow drag characteristics of suspensions are almost the same as those of water. While the heat transfer of MWNTs suspensions with high mass concentration or high fluid temperature is significantly enhanced. The fluid temperature does not affect flow drag characteristics but has great effect on the heat transfer characteristics. Nanometer characteristics are presented by suspensions with high MWNT mass concentration or high temperature on convective heat transfer.     

Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes

In the present study, the influences of twin-counter/co-twisted tapes (counter/co-swirl tape) on heat transfer rate (Nu), friction factor (f) and thermal enhancement index (η) are experimentally determined. The twin counter twisted tapes (CTs) are used as counter-swirl flow generators while twin co-twisted tapes (CoTs) are used as co-swirl flow generators in a test section. The tests are conducted using the CTs and CoTs with four different twist ratios (y/w = 2.5, 3.0, 3.5 and 4.0) for Reynolds numbers range between 3700 and 21,000 under uniform heat flux conditions. The experiments using the single twisted tape (ST) are also performed under similar operation test conditions, for comparison. The experimental results demonstrate that Nusselt number (Nu), friction factor (f) and thermal enhancement index (η) increase with decreasing twist ratio (y/w). The results also show that the CTs are more efficient than the CoTs for heat transfer enhancement. In the range of the present work, heat transfer rates in the tube fitted with the CTs are around 12.5–44.5% and 17.8–50% higher than those with the CoTs and ST, respectively. The maximum thermal enhancement indices (η) obtained at the constant pumping power by the CTs with y/w = 2.5, 3.0, 3.5 and 4.0, are 1.39, 1.24, 1.12 and 1.03, respectively, while those obtained by using the CoTs with the same range of y/w are 1.1, 1.03, 0.97 and 0.92, respectively. In addition, the empirical correlations of the heat transfer (Nu), friction factor (f) and thermal enhancement index (η) are also reported.     

Evaporation heat transfer of R-134a inside a microfin tube with different tube inclinations

An experimental investigation has been carried out to study the heat transfer characteristics during evaporation of R-134a inside a single helical microfin tube. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of −90° to +90° and four mass velocities of 53, 80, 107 and 136 kg/m² s for each tube inclination angle during evaporation of R-134a. The results demonstrate that the tube inclination angle, α, affects the boiling heat-transfer coefficient in a significant manner. For all refrigerant mass velocities, the best performing tube is that having inclination angle of α = +90°. The effect of tube inclination angle, α, on heat-transfer coefficient, h, is more prominent at low vapor quality and mass velocity. An empirical correlation has also been developed to predict the heat-transfer coefficient during flow boiling inside a microfin tube with different tube inclinations.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in small diameter tubes

The condensation heat transfer of pure refrigerants, R-22, R-134a and a binary refrigerant R-410A flowing in small diameter tubes was investigated experimentally. The condenser is a countflow heat exchanger which refrigerant flows in the inner tube and cooling water flows in the annulus. The heat exchanger is smooth, horizontal copper tube of 1.77, 3.36 and 5.35 mm inner diameter, respectively. The length of heat exchanger is 1220, 2660 and 3620 mm, respectively. The experiments were conducted at mass flux of 200–400 kg/m² s and saturation temperature of 40°C. The main results were summarized as follows: in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski and Wu and Little correlation. The new single-phase correlation based on the experimental data was proposed in this study. In case of two-phase flow, the condensation heat transfer coefficient of R-410A for three tubes was slightly higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. The condensation heat transfer coefficient for R-22, R-134a and R-410A increased with increasing mass flux and decreasing tube diameter. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensation heat transfer. Therefore, the new condensation heat transfer correlation based on the experimental data was proposed in the present study.     

Turbulent heat transfer in circular tube flows of viscoelastic fluids

The effects of thermal entrance length, polymer degradation and solvent chemistry were found to be critically important in the determination of the drag and heat transfer behavior of viscoelastic fluids in turbulent pipe flow. The minimum heat transfer asymptotic values in the thermally developing and in the fully developed regions were experimentally determined for relatively high concentration solutions of heat transfer resulting in the following correlations: $$\begin{gathered} j_H = 0.13\left( {\frac{x}{d}} \right)^{ - 0.24} \operatorname{Re} _a^{ - 0.45} thermally developing region \hfill \\ x/d< 450 \hfill \\ j_H = 0.03 \operatorname{Re} _a^{ - 0.45} thermally developed region \hfill \\ x/d< 450 \hfill \\ \end{gathered} $$ For dilute polymer solutions the heat transfer is a function ofx/d, the Reynolds number and the polymer concentration. The Reynolds analogy between momentum and heat transfer which has been widely used in the literature for Newtonian fluids is found not to apply in the case of drag-reducing viscoelastic fluids.     

Laminar heat transfer characteristics of internally finned tube with sinusoidal wavy fin

Comparative numerical study of laminar heat transfer characteristics of annular tubes with sinusoidal wavy fins has been conducted both experimentally and numerically with Re = 299–1,475. The uniform heat flux is imposed on the tube outside wall surface. Two tube materials (copper and stainless steel) are considered. It is found that the fluid temperature profile is not linear but convex along the flow direction due to the axial heat conduction in tube wall, and the effects of axial heat conduction on the heat transfer decreases with an increase in Reynolds number or decrease in tube wall thermal conductivity. The axial distributions of local Nusselt number could reach periodically fully developed after 3–5 cycles. The convectional data reduction method based on the traditional method should be improved for tube with high thermal conductivity or low Reynolds numbers, Otherwise, the heat transfer performance of internally finned tube may be underestimated.     

Heat transfer correlation for boiling flows in small rectangular horizontal channels with low aspect ratios

In the present experimental study, a correlation is proposed to represent the heat transfer coefficients of the boiling flows through horizontal rectangular channels with low aspect ratios. The gap between the upper and the lower plates of each channel ranges from 0.4 to 2 mm while the channel width being fixed to 20 mm. Refrigerant 113 was used as the test fluid. The mass flux ranges from 50 to 200 kg/m² s and the channel walls were uniformly heated up to 15 kW/m². The quality range covers from 0.15 to 0.75 and the flow pattern appeared to be annular. The modified Lockhart–Martinelli correlation for the frictional pressure drop was confirmed to be within an accuracy of ±20%. The heat transfer coefficients increase with the mass flux and the local quality; however the effect of the heat flux appears to be minor. At the low mass flux condition, which is more likely to be with the smaller gap size, the heat transfer rate is primarily controlled by the liquid film thickness. A modified form of the enhancement factor F for the heat transfer coefficient in the range of Re_LF200 well correlates the experimental data within the deviation of ±20%. The Kandlikar's flow boiling correlation covers the higher mass flux range (Re_LF>200) with 10.7% mean deviation.