scholarly journals Thermo-Hydraulic Performance of Solar Air Collectors with Artificially Roughened Absorbers: A Comparative Review of Semi-Empirical Models

Energies ◽  
2020 ◽  
Vol 13 (14) ◽  
pp. 3536 ◽  
Author(s):  
António Araújo
Keyword(s):  
Heat Transfer ◽  
Thermal Characteristics ◽  
Operating Conditions ◽  
Roughness Parameters ◽  
Energy Characteristics ◽  
Effective Roughness ◽  
Air Stream ◽  
Comparative Review ◽  
Efficiency And Effectiveness ◽  
Semi Empirical

Due to the poor thermal characteristics of the air, the absorber roughness of solar air collectors is commonly artificially increased in order to enhance the heat transfer to the air stream. However, this is also accompanied by an undesirable increase in the pumping power due to increased friction losses. As a result, several authors have experimentally investigated several ways of maximizing the heat transfer while minimizing the friction losses of different absorbers, resulting in the development of semi-empirical functions relating the Nusselt number (a measure of heat transfer) and the friction factor (a measure of friction losses) to the Reynolds number and the roughness parameters considered for each absorber. The present paper reviews, considering the publications from the last ten years, these semi-empirical functions. Moreover, the optimum roughness parameters and operating conditions of the absorbers were estimated by finding the maximum values of two performance parameters (the thermo-hydraulic efficiency and effectiveness), calculated using the semi-empirical functions, in order to classify the absorbers in terms of their energy characteristics. This approach proves to be a rather effective way of optimizing the roughness characteristics of solar air collector absorbers. It is also concluded that, considering the range of absorbers analyzed here, generally, multiple V-shaped ribs with gaps provide the most effective roughness geometry.

Experimental Investigation of Heat Transfer From a Discretely Heated Protruding Pedestal to a Single Round Impinging Air Jet

2004 ◽  
Author(s):  
Amy S. Fleischer ◽  
Sharareh R. Nejad
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Heated Surface ◽  
Operating Conditions ◽  
Surface Geometry ◽  
Stagnation Region ◽  
Exit Surface ◽  
Air Jet ◽  
Air Stream ◽  
Impingement Surface

An experimental investigation to understand the influence of the impingement surface geometry on the heat transfer from a discretely heated surface to a single round impinging jet is conducted. In this study, heat transfer at the stagnation region of a discretely heated pedestal protruding into an air stream is compared to the heat transfer on a discretely heated flat plate to determine the influence of impingement surface geometry on heat transfer for various Reynolds numbers, jet diameters and jet exit-surface spacings. The round jet issues from a tube of diameter 3.5 mm, 9.5 mm or 21 mm at jet exit-to-surface distances of 2–5 diameters with Re = 10,000–30,000. Under all operating conditions, the presence of a protruding pedestal is found to increase heat transfer.

A Physics-Based Heat Transfer Analysis for Aerospace Ball Thrust Bearings

2005 ◽  
Author(s):  
Sean McCutchan ◽  
Roger M. Barnsby
Keyword(s):  
Heat Transfer ◽  
Thermal Characteristics ◽  
New Physics ◽  
Operating Conditions ◽  
Heat Transfer Analysis ◽  
Critical Parameters ◽  
Flow Equations ◽  
Bearing Ring ◽  
Lubricant Property ◽  
Wide Range

Component temperatures have a significant influence on ball bearing life and durability. Critical parameters such as internal clearance, contact stress and oil film thickness are influenced by the thermal characteristics of a bearing. Previous heat transfer analyses that predict bearing ring temperatures have been typically based on empirical data. A new physics-based analysis has been developed which is derived from fundamental heat transfer and fluid flow equations. Gyroscopic torque and churning forces are balanced by oil shearing contact traction, using recent developments in new life theory [1] and lubricant property models [2]. Bearing ring temperatures and heat generation calculated by the new analysis have been successfully correlated with test data over a wide range of bearing sizes and operating conditions.

Investigation on the Lubrication Performances and Thermal Characteristics of the Tapered Roller Bearing

2017 ◽  
Author(s):  
Yunjia Zhang ◽  
Dengfang Ruan
Keyword(s):  
Heat Transfer ◽  
Roller Bearing ◽  
Thermal Characteristics ◽  
Simulation Models ◽  
Operating Conditions ◽  
Dual Clutch Transmission ◽  
Ansys Fluent ◽  
Tapered Roller Bearing ◽  
Tapered Roller ◽  
Transfer Properties

In this paper, the tapered roller bearing supported on the output shaft of the dual clutch transmission was studied. During the operating process of the DCT (Dual Clutch Transmission) gearbox, the heat generation of the bearing is very large due to the large operating load and high operating speed, which will easily result in bearing failure, such as pitting and abrasion, so it is necessary to investigate the lubrication performances and thermal characteristics of the tapered roller bearing. The simulation models considering or not considering the roller’s spinning (the rollers rotating on their own axles) were established based on Ansys Fluent software. The influences of the roller’s spinning on the lubrication performances of the bearing were analyzed. Furthermore, the transit heat transfer properties of the bearing were simulated and analyzed. The roller’s spinning and transit heat boundary specification were realized by using UDF (user-defined functions). At the same time, the lubrication performances and heat transfer properties of the bearing with different operating conditions are presented and analyzed.

scholarly journals Improvement of methods for calculating thermal characteristics of loop air heaters

2021 ◽  
Vol 1 (8 (109)) ◽  
pp. 36-43
Author(s):  
Volodymyr Yurko ◽  
Anton Ganzha ◽  
Oleksandra Tarasenko ◽  
Larysa Tiutiunyk
Keyword(s):  
Heat Transfer ◽  
Particle Size ◽  
Dust Particle ◽  
Thermal Characteristics ◽  
Heating Surface ◽  
Environmental Safety ◽  
Operating Conditions ◽  
Thermal Calculation ◽  
Transfer Coefficients ◽  
Air Heater

Utilization of heat from gases leaving the waelz process is a promising way to increase its energy efficiency and environmental safety. Taking into account the gas dustiness, the most rational is the use of a loop air heater, which is a multi-pass and multi-section heat exchanger with a complex mixed scheme of coolant movement. In modern conditions, when the methods and means of calculation of such devices are simplified, the task of obtaining improved methods and means of calculation, determining the efficiency and reliability of their work is relevant. Two mathematical models of the process of heat transfer and hydroaerodynamics in a multi-pass tubular air heater with a cross-circuit of coolants are used. The developed models for the loop air heater are based on the main methods of thermal calculation: a simpler method of correction factor to the average logarithmic temperature pressure and a discrete P-NTU method, which allows obtaining local thermal characteristics of the surface. Diagrams of distribution of heat transfer coefficients, heat transfer, local temperatures of flue gases, air and pipe walls are constructed. The influence of dust and dust particle size on heat transfer is determined. When the flue gas dust is 50 g/Nm3 and with a dust particle size of 1 μm, the heat transfer coefficient increases by 12 %. The application of the air heater design with different schemes of coolant movement is substantiated. The developed universal methods allow determining the thermal productivity of heat exchangers and obtaining the distribution of local temperature characteristics on the heating surface. It is also possible to identify places of possible overheating of the heat exchange surface and the course of corrosion processes, taking into account the design of recuperators, operating conditions, operating modes and different schemes of coolant movement

A Semi Empirical Methodology for Performance Estimation of a Double Pipe Finned Heat Exchanger

2008 ◽  
Author(s):  
G. Arvind Rao ◽  
Yeshyahou Levy
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
Operating Conditions ◽  
Performance Estimation ◽  
Wide Range ◽  
Double Pipe ◽  
Semi Empirical ◽  
Pipe Heat

Finned tubes are one of the most widely used means of passively enhancing the heat transfer in circular tubes. Many investigators have proposed different correlations for predicting the performance of such heat exchangers based on their experimental investigations. However, the practical usage of such correlations is limited because of the variety of parameters that can influence Nusselt number and friction factor. Most of the correlations either have been developed with limited databases, or are geometry specific. Using CFD for analyzing performance of such heat exchangers is very computational intensive and hence cannot practically be applied for design optimization purposes. On the other hand, empirical correlations have many limitations in terms of their applicability. The objective of the present article is to present a physically based model for evaluating heat transfer and frictional loss for an internally and / or externally finned double pipe heat exchanger that can be applied in a wide range of operating conditions of practical importance. This paper describes a simple semi-empirical-numerical methodology to evaluate heat transfer and pressure drop characteristics in a finned tube heat exchanger with internal and/or external fins. Conduction and turbulent forced convection are the prominent modes of heat transfer. In order to resolve the operational characteristics of double pipe finned heat exchangers, a numerical methodology is presented which uses well known existing correlations for flow in a smooth pipe and flow over a flat plate. The method of successive substitution is used to solve the problem numerically. The proposed methodology is applied to some simple cases and the results compare well with existing data and correlations available in the literature. It is found that the addition of fins to such double pipe heat exchangers reduce the Nusselt Number; however the corresponding heat transfer rate is enhanced owing to the increase in the overall heat transfer area.

Experimental Study on Thermal Characteristics of Finned Coil LHSU Using Paraffin as Phase Change Material

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Guansheng Chen ◽  
Nanshuo Li ◽  
Huanhuan Xiang ◽  
Fan Li
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Phase Change ◽  
Phase Change Material ◽  
Water Flow ◽  
Water Flow Rate ◽  
Thermal Characteristics ◽  
Heat Transfer Fluid ◽  
Latent Heat Storage ◽  
Change Material

It is well known that attaching fins on the tubes surfaces can enhance the heat transfer into and out from the phase change materials (PCMs). This paper presents the results of an experimental study on the thermal characteristics of finned coil latent heat storage unit (LHSU) using paraffin as the phase change material (PCM). The paraffin LHSU is a rectangular cube consists of continuous horizontal multibended tubes attached vertical fins at the pitches of 2.5, 5.0, and 7.5 mm that creates the heat transfer surface. The shell side along with the space around the tubes and fins is filled with the material RT54 allocated to store energy of water, which flows inside the tubes as heat transfer fluid (HTF). The measurement is carried out under four different water flow rates: 1.01, 1.30, 1.50, and 1.70 L/min in the charging and discharging process, respectively. The temperature of paraffin and water, charging and discharging wattage, and heat transfer coefficient are plotted in relation to the working time and water flow rate.

scholarly journals Experimental performance comparison between circular and elliptical tubes in evaporative condensers

2021 ◽  
Author(s):  
Giuseppe Starace ◽  
Lorenzo Falcicchia ◽  
Pierpaolo Panico ◽  
Maria Fiorentino ◽  
Gianpiero Colangelo
Keyword(s):  
Heat Transfer ◽  
Experimental Approach ◽  
Fluid Dynamic ◽  
Major Axis ◽  
Performance Comparison ◽  
Operating Conditions ◽  
Condensation Temperature ◽  
Air Cooled Condensers ◽  
Reduced Scale ◽  
Dynamic Phenomena

AbstractIn refrigeration systems, evaporative condensers have two main advantages compared to other condensation heat exchangers: They operate at lower condensation temperature than traditional air-cooled condensers and require a lower quantity of water and pumping power compared to evaporative towers. The heat and mass transfer that occur on tube batteries are difficult to study. The aim of this work is to apply an experimental approach to investigate the performance of an evaporative condenser on a reduced scale by means of a test bench, consisting of a transparent duct with a rectangular test section in which electric heaters, inside elliptical pipes (major axis 32 mm, minor axis 23 mm), simulate the presence of the refrigerant during condensation. By keeping the water conditions fixed and constant, the operating conditions of the air and the inclination of the heat transfer geometry were varied, and this allowed to carry out a sensitivity analysis, depending on some of the main parameters that influence the thermo-fluid dynamic phenomena, as well as a performance comparison. The results showed that the heat transfer increases with the tube surface exposed directly to the air as a result of the increase in their inclination, that has been varied in the range 0–20°. For the investigated conditions, the average increase, resulting by the inclination, is 28%.

scholarly journals Intelligent long-term performance analysis in power electronics systems

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Saeed Peyghami ◽  
Tomislav Dragicevic ◽  
Frede Blaabjerg
Keyword(s):  
Performance Indicator ◽  
Thermal Characteristics ◽  
Operating Conditions ◽  
System Level ◽  
Power Electronic ◽  
Reliability Modeling ◽  
Artificial Neural Network Ann ◽  
Long Term Performance ◽  
Term Performance

AbstractThis paper proposes a long-term performance indicator for power electronic converters based on their reliability. The converter reliability is represented by the proposed constant lifetime curves, which have been developed using Artificial Neural Network (ANN) under different operating conditions. Unlike the state-of-the-art theoretical reliability modeling approaches, which employ detailed electro-thermal characteristics and lifetime models of converter components, the proposed method provides a nonparametric surrogate model of the converter based on limited non-linear data from theoretical reliability analysis. The proposed approach can quickly predict the converter lifetime under given operating conditions without a further need for extended, time-consuming electro-thermal analysis. Moreover, the proposed lifetime curves can present the long-term performance of converters facilitating optimal system-level design for reliability, reliable operation and maintenance planning in power electronic systems. Numerical case studies evaluate the effectiveness of the proposed reliability modeling approach.

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