Correlations and Theory for Microchannel Condensation

2008 ◽  
Author(s):  
Qian Su ◽  
Guang Xu Yu ◽  
Hua Sheng Wang ◽  
John W. Rose
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Theoretical Approach ◽  
Annular Flow ◽  
Fair Agreement ◽  
Calculation Methods ◽  
Fluid Property ◽  
The Heat Transfer Coefficient

In recent years several correlations have been proposed for calculating the heat-transfer coefficient during condensation in circular and non-circular channels of typical dimension around 1 mm where surface tension effects are important and correlations for larger diameter channels are inappropriate. A wholly theoretical approach applicable to annular flow has also been proposed. The correlations are all based on data for R134a, while the theory is applicable to any fluid. In this paper comparisons are made between the correlations for R134a and ammonia; plots based on theory are also included. Fair agreement is seen between all calculation methods for R134a but wide differences are seen for ammonia indicating that the correlations, based only on one fluid, do not capture the fluid property dependence accurately.

scholarly journals Comparative study of flow condensation in conventional and small diameter tubes

2012 ◽  
Vol 33 (2) ◽  
pp. 67-83 ◽  
Author(s):  
Dariusz Mikielewicz ◽  
Rafał Andrzejczyk
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Structure ◽  
Annular Flow ◽  
Flow Boiling ◽  
Small Diameter ◽  
Bubble Generation ◽  
Flow Condensation ◽  
The Heat Transfer Coefficient

Abstract Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena. Their description however with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where the bubble generation is not present. Similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such case the heat transfer coefficient is primarily dependent on the convective mechanism. In the paper a method developed earlier by the first author is applied to calculations of heat transfer coefficient for inside tube condensation. The method has been verified using experimental data from literature on several fluids in different microchannels and compared to three well established correlations for calculations of heat transfer coefficient in flow condensation. It clearly stems from the results presented here that the flow condensation can be modeled in terms of appropriately devised pressure drop.

scholarly journals Modeling heat exchange in recovery direct-flow boiler of auxiliary power plant based on organic Rankine cycle

2021 ◽  
Vol 2021 (1) ◽  
pp. 65-73
Author(s):  
Nikolay Nikitovich Panasenko ◽  
Pavel Victorovich Yakovlev ◽  
Mark Alekseevich Peretyatko ◽  
Sabina Andreevna Peretyatko
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Heat Transfer Coefficient ◽  
Physical Properties ◽  
Transfer Coefficient ◽  
Organic Rankine Cycle ◽  
Rankine Cycle ◽  
Heat Of Vaporization ◽  
Auxiliary Power ◽  
The Heat Transfer Coefficient

Currently, there are many ways to improve the energy efficiency of different technological processes. The article presents the results of heat transfer simulating in a recovery direct-flow boiler of a ship auxiliary power plant using the organic Rankine cycle. There has been illustrated the basic circuit of the organic Rankine cycle unit. The parameters of the studied working bodies (pentane, ethanol, methanol) have been given. The process of the coolant boiling (for methanol) is considered, the process (5 s) of the coolant boiling from the liquid state to the steady state is modeled. There are presented the graphs of the dependence of the heat transfer coefficient on the length of the evaporator tube, on the specific heat of vaporization, on the surface tension, on the physical properties of the heat carrier material. There have been summarized the results of studying the dependence of the heat transfer coefficient during organic coolant boiling on such physical properties of the coolant as heat of vaporization and surface tension. Obtaining a numerical dependence that allows to calculate the heat transfer coefficient during the boiling of organic heat carriers, taking into account the physical properties of these heat carriers has been analyzed. The study was carried out by creating a numerical model of the evaporator in the ANSYS Fluent software. It has been found out that with the increasing ratio of the surface tension force to the specific heat of vaporization, the heat transfer coefficient increases. The empirical equation was also obtained for calculating the heat transfer coefficient during the organic heat carrier boiling.

Analytical Modeling of Annular Flow Boiling Heat Transfer in Mini- and Microchannel Heat Sinks

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
A. Megahed ◽  
I. Hassan
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Analytical Model ◽  
Boiling Heat Transfer ◽  
Annular Flow ◽  
Flow Boiling ◽  
Heat Sinks ◽  
Flow Boiling Heat Transfer ◽  
The Heat Transfer Coefficient

An analytical model is proposed to predict the flow boiling heat transfer coefficient in the annular flow regime in mini- and microchannel heat sinks based on the separated model. The modeling procedure includes a formulation for determining the heat transfer coefficient based on the wall shear stress and the local thermophysical characteristics of the fluid based on the Reynolds’ analogy. The frictional and acceleration pressure gradients within the channel are incorporated into the present model to provide a better representation of the flow conditions. The model is validated against collected data sets from the literature produced by different authors under different experimental conditions, different fluids, and with mini- and microchannels of hydraulic diameters falling within the range of 92–1440 μm. The accuracy between the experimental and predicted results is achieved with a mean absolute error of 10%. The present analytical model can correctly predict the different trends of the heat transfer coefficient reported in the literature as a function of the exit quality. The predicted two-phase heat transfer coefficient is found to be very sensitive to changes in mass flux and saturation temperature. However, it is found to be mildly sensitive to the change in heat flux.

Study for Bubble Dynamics of Nucleate Boiling in Narrow Channels

2010 ◽  
Vol 123-125 ◽  
pp. 499-502 ◽  
Author(s):  
Lei Guo ◽  
Shu Sheng Zhang ◽  
Lin Cheng
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Contact Angle ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Great Influence ◽  
Nucleate Boiling ◽  
Narrow Channels ◽  
The Heat Transfer Coefficient

Two different types of channels are investigated which have I- and Z-shaped cross-sections with a width of 2mm. Using the numerical simulation method, the influence of wall contact angle to the process of bubble generating and growth up is studied, and the relationship between different channel shapes and pressure drop is also investigated. In the calculation process, the effects of gravity, surface tension and wall adhesion are taken into account. It is found that wall contact angle has a great influence to the morphology of bubbles. The smaller the wall contact angle is, the rounder the bubbles are, and the shorter the bubbles take to departure from the wall, otherwise, the bubbles are more difficult to depart. The variation of contact angle also has effect upon the heat transfer coefficient, the greater the wall contact angle is, the larger bubble-covered area is, thus the wall thermal resistance gets higher, and the heat transfer coefficient becomes lower. The role of surface tension in the process of boiling heat transfer is much larger than the gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance the heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of I-shaped channels, while the pressure drop of the former is obviously higher.

Heat Transfer Behavior of the Boundary Layer Near Surfaces in Annular Flow of High Capacity Canned Motor Pump

2016 ◽  
Author(s):  
Shengde Wang ◽  
Guohu Luo ◽  
Hong Shen ◽  
Zhenqiang Yao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Annular Flow ◽  
High Capacity ◽  
Heat Transfer Behavior ◽  
Transfer Behavior ◽  
Canned Motor ◽  
The Heat Transfer Coefficient

As significant fluid machinery, canned motor pumps are widely applied in industrial field. The typical characteristic of canned motor pump is that the fluid comes into the narrow gap and affects the performance of canned motor. The coolant flow in the narrow annular gap between rotor and stator cans belongs to Taylor-Couette-Poiseuille flow which has been investigated for a long time while the thermal design is a key function of internal narrow gap annular flow of canned motor. However, the temperature distribution prediction of canned motor deviates from the experiments, especially in the high-capacity canned motor due to the large shear rate of fluid and eddy-current loss of motor’s can. According to the researcher’s work, the significant work lies in the heat transfer coefficient that different researchers give various numerical prediction and experimental measurement. It brings big challenge in thermal design of high-capacity canned motor pump. In this paper, the author focuses on the reason why the heat transfer coefficient is remarkably lower than that other’s forecast. In this paper, the heat transfer behavior of the boundary layer near surfaces in the annular flow is investigated by using the commercial fluid dynamic (CFD) method. Firstly, the Naiver-Stocks (N-S) equations and energy conservation equation are employed for modeling the flow and heat transfer behavior, and the k-ω turbulent model is used for solving the flow control equations. Secondly, the fluid domain is described by a simplified geometrical model: two concentric cylinders with finite gap length. Thirdly, numerical approach is used to analyze the subject with tools of Solidworks, ICEM CFD and Ansys Fluent. Two parameters are analyzed in the research, namely the rotating speed and the wall heat flux, without considering the fluid viscous dissipation and thermal contact resistance. Numerical simulation results indicate that Taylor vortex exists in the flow regime, and the temperature distribution is affected by both the rotating speed and the wall heat flux, named thermal barrier effect under large heat flux condition. The thermal barrier effect lies in that the temperature gradient of interface decreases compared to the peak value of temperature gradient near the surface, so that the heat transfer coefficient is reduced remarkably. This effect leads to the temperature prediction deviates from the experiment measurement.

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%.

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