scholarly journals Using Long Term Simulations to Understand Heat Transfer Processes during Steady Flow Conditions in Combined Sewers

Water ◽  
2021 ◽  
Vol 13 (4) ◽  
pp. 570
Author(s):  
Mohamad Abdel-Aal ◽  
Simon Tait ◽  
Mostafa Mohamed ◽  
Alma Schellart
Keyword(s):  
Heat Transfer ◽  
Heat Recovery ◽  
Predictive Accuracy ◽  
Flow Conditions ◽  
Empirical Relationships ◽  
Taylor Diagram ◽  
Physical Phenomena ◽  
The Heat Transfer Coefficient ◽  
Empirical Approaches

This paper describes a new heat transfer parameterisation between wastewater and in-sewer air based on understanding the physical phenomena observed in free surface wastewater and in-sewer air. Long-term wastewater and in-sewer air temperature data were collected and studied to indicate the importance of considering the heat exchange with in-sewer air and the relevant seasonal changes. The new parameterisation was based on the physical flow condition variations. Accurate modelling of wastewater temperature in linked combined sewers is needed to assess the feasibility of in-sewer heat recovery. Historically, the heat transfer coefficient between wastewater and in-sewer air has been estimated using simple empirical relationships. The newly developed parameterisation was implemented and validated using independent long-term flow and temperature datasets. Predictive accuracy of wastewater temperatures was investigated using a Taylor diagram, where absolute errors and correlations between modelled and observed values were plotted for different site sizes and seasons. The newly developed coefficient improved wastewater temperature modelling accuracy, compared with the older empirical approaches, which resulted in predicting more potential for heat recovery from large sewer networks. For individual locations, the RMSE between observed and predicted temperatures ranged between 0.15 and 0.5 °C with an overall average of 0.27 °C. Previous studies showed higher RMSE ranges, e.g., between 0.12 and 7.8 °C, with overall averages of 0.35, 0.42 and 2 °C. The new coefficient has also provided stable values at various seasons and minimised the number of required model inputs.

A Numerical Investigation of Nanofluids Thermal Behavior in Microchannels Under Laminar Flow Conditions

2015 ◽  
Author(s):  
I. P. Koronaki ◽  
M. T. Nitsas ◽  
Ch. A. Vallianos
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Laminar Flow ◽  
Volume Fraction ◽  
Constant Heat Flux ◽  
Flow Conditions ◽  
New Techniques ◽  
Pressure Drops ◽  
Nanoparticles Volume Fraction ◽  
The Heat Transfer Coefficient

Due to large amounts of heat flux developed in electronic devices, it is essential to propose and investigate effective mechanisms of cooling them. Although microchannels filled with flowing coolant are a geometry often met in such devices, new techniques need to be developed in order to increase their effectiveness. Recent studies on nanofluids, i.e. mixtures of nanometer size particles well-dispersed in a base fluid, have demonstrated their potential for augmenting heat transfer. In the present work the 2D steady state laminar flow of different nanofluids along a microchannel is examined. It is considered that the microchannel walls receive uniform and constant heat flux. The problem’s modelling has as parameters the volume fraction of nanoparticles ranging from 0 to 5% and Reynolds number varying between 50 and 500. The results of the problem’s numerical solution are used to calculate the heat transfer coefficient, the pressure drop along the microchannel and the destroyed exergy. It is found that heat transfer is enhanced due to the presence of nanoparticles. On the contrary, pressure drops faster due to nanofluids increased viscosity leading to more pump power needed. Finally, further exergy destruction is observed when nanoparticles volume fraction increases.

scholarly journals Determination of long-term permissible currents in wires of power supply systems of railways

2019 ◽  
Vol 78 (2) ◽  
pp. 90-95 ◽  
Author(s):  
E. P. FIGURNOV ◽  
Yu. I. ZHARKOV ◽  
V. I. KHARCHEVNIKOV
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Solar Radiation ◽  
Transfer Coefficient ◽  
Surface Area ◽  
Meteorological Conditions ◽  
The Body ◽  
Additional Heating ◽  
The Heat Transfer Coefficient

In the standard for contact wires made from copper and its alloys, the values of long-term permissible temperatures have significantly decreased. This requires recalculation of previously valid values of long-term permissible currents. Authors considered revised method for calculating the long-term permissible currents, based on a more rigorous consideration of the laws of heat transfer and experimental studies of the conditions of heating and cooling of shaped (contact) and stranded wires. Technique is based on heat balance conditions, using which the sources of greatest inaccuracies become such quantities as cooled surface area, influence of wind direction, meteorological conditions, laws of change in heat transfer coefficient, effect on additional heating of solar radiation. Deviations when these indicators are taken into account by existing methods can cause errors of 40 % or more. Formulas for calculating the actual outer surface of stranded and shaped wires are given. The inadmissibility of calculating the surface area of the wires by their reference diameter is noted. Updated law of the change in heat transfer coefficient for stranded and shaped wires, as well as the degree of its dependence on wind speed and cooled surface, is given based on a summary of extensive domestic and foreign research. It is shown that with the longitudinal direction of the wind, the reduction of this coefficient occurs to a lesser extent than has been assumed so far. Authors propose method for taking into account an increase in the heat transfer coefficient under meteorological conditions characteristic of ice formation. The heat transfer coefficient of shaped and stranded wires in no case can not be taken as for round pipes with smooth surface. Existing method of accounting for solar radiation, which influences the additional heating of wires, leads to an unjustified and repeated exaggeration of this effect, since previously only the radiation incident on the wire was taken into account in the calculations. According to the laws of heat transfer, the temperature of the irradiated body does not depend on the incident, but on the resulting radiation, defined as the difference between the radiations incident on the body and emitted by it in accordance with its temperature. A formula for accounting for such heat transfer is proposed. The above methodology and calculation formulas allow performing reasonable calculations to determine the long-term permissible currents of individual stranded and shaped wires, as well as the contact network as a whole.

scholarly journals Delineation of dew formation zones in Iran using long-term model simulations and cluster analysis

2021 ◽  
Vol 25 (9) ◽  
pp. 4719-4740
Author(s):  
Nahid Atashi ◽  
Dariush Rahimi ◽  
Victoria A. Sinclair ◽  
Martha A. Zaidan ◽  
Anton Rusanen ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Cluster Analysis ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Negative Trend ◽  
Balance Model ◽  
Formation Potential ◽  
Dew Formation ◽  
The Heat Transfer Coefficient

Abstract. Dew is a non-conventional source of water that has been gaining interest over the last two decades, especially in arid and semi-arid regions. In this study, we performed a long-term (1979–2018) energy balance model simulation to estimate dew formation potential in Iran aiming to identify dew formation zones and to investigate the impacts of long-term variation in meteorological parameters on dew formation. The annual average of dew occurrence in Iran was ∼102 d, with the lowest number of dewy days in summer (∼7 d) and the highest in winter (∼45 d). The average daily dew yield was in the range of 0.03–0.14 L m−2 and the maximum was in the range of 0.29–0.52 L m−2. Six dew formation zones were identified based on cluster analysis of the time series of the simulated dew yield. The distribution of dew formation zones in Iran was closely aligned with topography and sources of moisture. Therefore, the coastal zones in the north and south of Iran (i.e., Caspian Sea and Oman Sea), showed the highest dew formation potential, with 53 and 34 L m−2 yr−1, whereas the dry interior regions (i.e., central Iran and the Lut Desert), with the average of 12–18 L m−2 yr−1, had the lowest potential for dew formation. Dew yield estimation is very sensitive to the choice of the heat transfer coefficient. The uncertainty analysis of the heat transfer coefficient using eight different parameterizations revealed that the parameterization used in this study – the Richards (2004) formulation – gives estimates that are similar to the average of all methods and are neither much lower nor much higher than the majority of other parameterizations and the largest differences occur for the very low values of daily dew yield. Trend analysis results revealed a significant (p<0.05) negative trend in the yearly dew yield in most parts of Iran during the last 4 decades (1979–2018). Such a negative trend in dew formation is likely due to an increase in air temperature and a decrease in relative humidity and cloudiness over the 40 years.

An Experimental Investigation of Reflux Condensation Phenomena in Multiple U-Tubes With and Without Noncondensible Gas

2001 ◽  
Author(s):  
Moon-Hyun Chun ◽  
Kyung-Won Lee ◽  
In-Cheol Chu
Keyword(s):  
Heat Transfer ◽  
Atmospheric Condition ◽  
Flow Conditions ◽  
Transfer Coefficients ◽  
Flow Rates ◽  
Heat Transfer Coefficients ◽  
Condensation Mode ◽  
Series Of Experiments ◽  
Pure Steam ◽  
The Heat Transfer Coefficient

Abstract A series of experiments were performed to investigate the thermal-hydraulic phenomena inside U-tubes in a reflux condensation mode. A total of 512 data for local condensation heat transfer coefficients (108 for pure steam flow and 404 for steam-air flow conditions, respectively) have been obtained for various inlet flow rates of steam and air under atmospheric condition. A new correlation, which includes the effects of flow rates of steam and noncondensible gases (air) on the heat transfer coefficient and is applicable to the reflux condensation mode, has been developed using the concept of degradation factor based on the steam-air experimental results. In addition, the effects of multiple U-tubes with different lengths (i.e., two-long and two-short U-tubes) and noncondensible gases on the onset of flooding during a reflux condensation have been examined.

scholarly journals Термоэлектрический интенсификатор теплопередачи между двумя движущимися средами с различной температурой

2019 ◽  
Vol 53 (7) ◽  
pp. 883
Author(s):  
Д.К. Кадирова
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Calculated Data ◽  
Flow Conditions ◽  
Different Temperatures ◽  
Media Flows ◽  
Moving Media ◽  
The Heat Transfer Coefficient ◽  
The Heat Balance

The patented thermoelectric intensifier of heat transfer between two moving media with different temperatures was investigated. In its design, to increase the intensity of heat exchange between media, the use of fan units is provided. A mathematical model of the device was developed, based on the solution of the heat balance equation for media flows in transport zones for forward flow conditions. The calculated data are presented in the form of the dependence of the temperature change of moving media along the length of the intensifier for different values ​​of the heat transfer coefficient between the junctions of the thermoelectric battery and the air medium in the gap of the device. It has been established that the larger the temperature difference between the inlet media, the more abrupt the dependence of the limiting length of the thermoelectric battery on the heat transfer coefficient will be.

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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