In-Tube Heat Transfer for Skewed Inlet Flow Caused by Competition Among Tubes Fed by the Same Plenum

1983 ◽  
Vol 105 (4) ◽  
pp. 870-877 ◽  
Author(s):  
M. Molki ◽  
E. M. Sparrow
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Test Section ◽  
Separation Distance ◽  
Axial Distribution ◽  
Naphthalene Sublimation ◽  
Flow Imbalance ◽  
A New Technique ◽  
The Heat Transfer Coefficient

Measurements were made of the axial and circumferential distributions of the heat transfer coefficient in a tube in which the entering airflow is highly skewed. The skewness was caused by competition between the test section tube and a parallel tube which draws air from the same plenum chamber. For each of several fixed Reynolds numbers in the test section tube, the flow imbalance between the competing tubes was varied parametrically (up to a factor of eighteen), as was the center-to-center separation distance between the tubes (separation = 1.5, 3, and 4.5 times the tube diameter). Measurements were also made of the pressure drop, and a visualization technique was employed to examine the pattern of fluid flow. Practically significant effects of the flow imbalance on the axial distribution of the heat transfer coefficient were encountered only at the smallest of the investigated intertube spacings. Even for that case, the effects were moderate; for example, the imbalance-related changes for an imbalance ratio of two did not exceed 7 percent. The experiments involved naphthalene sublimation, and a new technique was developed for coating the inside surface of a tube with naphthalene.

Falling Film and Spray Evaporation Enhancement Using an Applied Electric Field

2000 ◽  
Vol 122 (4) ◽  
pp. 741-748 ◽  
Author(s):  
J. Darabi ◽  
M. M. Ohadi ◽  
S. V. Desiatoun
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Electric Field ◽  
Transfer Coefficient ◽  
Test Section ◽  
Applied Electric Field ◽  
Falling Film ◽  
Electrode Gap ◽  
Maximum Enhancement ◽  
The Heat Transfer Coefficient

The effect of an electric field on the falling-film evaporation of refrigerant R-134a on a vertical plate and three commercially available tubes was investigated experimentally. The plate test section was 25.4 mm wide and 76.2 mm long, and each tube test section was 19 mm in diameter and 140 mm long. Experiments were conducted in both falling film and spray evaporation modes. The effects of various parameters such as heat flux, refrigerant flow rate, electrode gap, and applied voltage were investigated. It was found that in the presence of an applied electric field, the maximum enhancement in the heat transfer coefficient for both falling film and spray evaporation modes on a plate was nearly the same. A maximum enhancement of fourfold in the heat transfer coefficient with the plate, 90 percent with the smooth tube, 110 percent with the Turbo BIII, and 30 percent with 19 fpi tube were obtained. The electrohydrodynamic power consumption in all cases was less than 0.12 percent of the total energy exchange rate in the test section. [S0022-1481(00)03003-6]

Effect of By-Pass on the Heat Transfer Coefficient During Density Wave Oscillations in a Horizontal Mini-Channel

2018 ◽  
Author(s):  
Maria Fernandino ◽  
Carlos A. Dorao
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Test Section ◽  
Density Wave ◽  
Two Phase ◽  
The Stability ◽  
Density Wave Oscillations ◽  
Cold Liquid ◽  
The Heat Transfer Coefficient

Two phase flow instabilities and in particular density wave oscillations, DWO, are strongly dependent on the internal and external characteristics of the system. Although significant work has been done investigating the characteristics of the stability of the oscillations, the effect of the oscillations on the heat transfer coefficient demands further research. In this work, the influence of a parallel bypass to the test section on the heat transfer coefficient during density wave oscillations is studied. It is observed that in the case of small amplitude DWO the influence of the bypass is negligible, while for the case of large amplitude DWO that reach conditions of flow reversal the heat transfer coefficient can be enhanced. This fact is attributed to cold liquid entering at the outlet of the test section from the bypass preventing the dryout of the wall at high qualities.

Flow Boiling Study in Mini-Channels

2006 ◽  
Author(s):  
Francesc Madrid ◽  
Nadia Caney ◽  
Philippe Marty
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Test Section ◽  
Heat Exchangers ◽  
Flow Boiling ◽  
Two Phase ◽  
Mini Channels ◽  
Vapour Quality ◽  
The Heat Transfer Coefficient

Mini-channel heat exchangers improve thermal performance in comparison to conventional macro-channel heat exchangers, being highly efficient, compact and requiring low fluid mass. However, classical correlations for two-phase flow in macro-channels fail in predicting the heat transfer coefficient in mini-channels. Therefore, new studies are needed in order to provide better knowledge on flow boiling phenomena in confined spaces. The proposed paper presents an experimental study on two-phase vertical flow boiling in mini-channels. The aim of this work is to determine the heat transfer coefficient and to study the pressure drop in a mini-channel heat exchanger (hydraulic diameter of 840μm) in order to obtain better understanding of the flow boiling mechanisms. A vertical upward flow test section is connected to a primary HFE-7100 circuit. A preheater imposes a given sub-cooled fluid temperature or a given two-phase vapour quality at the inlet. Downstream in the test loop, the fluid is condensed and pumped again into the test section. The pressure drop and the heat transfer coefficient in the test section have been measured for a variety of conditions. Different heat flux, inlet vapour quality and mass flow rate values have been tested. For the heat transfer coefficient, a correlating model is proposed as a function of the superficial velocity. This parameter appears to be much more appropriate than the vapour quality or the mass flow rate for dry-out occurrence prediction. A single critical velocity value has been found.

Experimental Investigation on Condensation Heat Transfer Coefficient Inside Multi-Port Minichannels

2003 ◽  
Author(s):  
A. Cavallini ◽  
G. Censi ◽  
D. Del Col ◽  
L. Doretti ◽  
G. A. Longo ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Internal Heat ◽  
Experimental Information ◽  
Tube Wall Temperature ◽  
Wilson Plot ◽  
A New Technique ◽  
The Heat Transfer Coefficient ◽  
Internal Heat Transfer

Very little experimental information is available in the open literature about condensation inside minichannels. Most of the experimental work has been carried out by using the Wilson plot technique. This method is simple to implement because it does not require the direct measurement of the tube wall temperature. However it becomes inaccurate when a small thermal resistance is present on the test side as compared to the opposite (cooling) side, which is actually the case with a multichannel tube at high values of the internal heat transfer coefficient. In fact, in a multi-port tube internal webs work as fins, and their efficiency is close to unity; thus the internal heat transfer area is higher than the external one. In this paper a new technique to measure the heat transfer coefficient during condensation inside a multi-port extruded minichannel tube is presented. Some R134a preliminary data is also reported.

Evaporation of lignin-lean black liquor

TAPPI Journal ◽  
2015 ◽  
Vol 14 (7) ◽  
pp. 441-450
Author(s):  
HENRIK WALLMO, ◽  
ULF ANDERSSON ◽  
MATHIAS GOURDON ◽  
MARTIN WIMBY
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Black Liquor ◽  
Salt Content ◽  
Solubility Limit ◽  
Lignin Removal ◽  
Kraft Black Liquor ◽  
Black Liquors ◽  
The Heat Transfer Coefficient

Many of the pulp mill biorefinery concepts recently presented include removal of lignin from black liquor. In this work, the aim was to study how the change in liquor chemistry affected the evaporation of kraft black liquor when lignin was removed using the LignoBoost process. Lignin was removed from a softwood kraft black liquor and four different black liquors were studied: one reference black liquor (with no lignin extracted); two ligninlean black liquors with a lignin removal rate of 5.5% and 21%, respectively; and one liquor with maximum lignin removal of 60%. Evaporation tests were carried out at the research evaporator in Chalmers University of Technology. Studied parameters were liquor viscosity, boiling point rise, heat transfer coefficient, scaling propensity, changes in liquor chemical composition, and tube incrustation. It was found that the solubility limit for incrustation changed towards lower dry solids for the lignin-lean black liquors due to an increased salt content. The scaling obtained on the tubes was easily cleaned with thin liquor at 105°C. It was also shown that the liquor viscosity decreased exponentially with increased lignin outtake and hence, the heat transfer coefficient increased with increased lignin outtake. Long term tests, operated about 6 percentage dry solids units above the solubility limit for incrustation for all liquors, showed that the heat transfer coefficient increased from 650 W/m2K for the reference liquor to 1500 W/m2K for the liquor with highest lignin separation degree, 60%.

scholarly journals Systematic heat transfer measurements in highly viscous binary fluids

2021 ◽  
Author(s):  
Ann-Christin Fleer ◽  
Markus Richter ◽  
Roland Span
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Volatile Component ◽  
Test Tube ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Low Viscosity ◽  
The Heat Transfer Coefficient

AbstractInvestigations of flow boiling in highly viscous fluids show that heat transfer mechanisms in such fluids are different from those in fluids of low viscosity like refrigerants or water. To gain a better understanding, a modified standard apparatus was developed; it was specifically designed for fluids of high viscosity up to 1000 Pa∙s and enables heat transfer measurements with a single horizontal test tube over a wide range of heat fluxes. Here, we present measurements of the heat transfer coefficient at pool boiling conditions in highly viscous binary mixtures of three different polydimethylsiloxanes (PDMS) and n-pentane, which is the volatile component in the mixture. Systematic measurements were carried out to investigate pool boiling in mixtures with a focus on the temperature, the viscosity of the non-volatile component and the fraction of the volatile component on the heat transfer coefficient. Furthermore, copper test tubes with polished and sanded surfaces were used to evaluate the influence of the surface structure on the heat transfer coefficient. The results show that viscosity and composition of the mixture have the strongest effect on the heat transfer coefficient in highly viscous mixtures, whereby the viscosity of the mixture depends on the base viscosity of the used PDMS, on the concentration of n-pentane in the mixture, and on the temperature. For nucleate boiling, the influence of the surface structure of the test tube is less pronounced than observed in boiling experiments with pure fluids of low viscosity, but the relative enhancement of the heat transfer coefficient is still significant. In particular for mixtures with high concentrations of the volatile component and at high pool temperature, heat transfer coefficients increase with heat flux until they reach a maximum. At further increased heat fluxes the heat transfer coefficients decrease again. Observed temperature differences between heating surface and pool are much larger than for boiling fluids with low viscosity. Temperature differences up to 137 K (for a mixture containing 5% n-pentane by mass at a heat flux of 13.6 kW/m2) were measured.

Sign in / Sign up

Export Citation Format

Share Document