Bifurcation Analysis for Horizontal Longitudinal Fins Under Multi-Boiling Conditions

2002 ◽  
Author(s):  
Rizos N. Krikkis ◽  
Stratis V. Sotirchos ◽  
Panagiotis Razelos
Keyword(s):  
Heat Transfer ◽  
Bifurcation Analysis ◽  
Solution Structure ◽  
Heat Dissipation ◽  
System Analysis ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Constant Thickness ◽  
Operating Variables ◽  
Numerical Bifurcation

A numerical bifurcation analysis is carried out in order to determine the solution structure of a fin subject to multi-boiling heat transfer mode. The thermal analysis can no longer performed independently of the working fluid since the heat transfer coefficient is temperature dependent and includes the nucleate, the transition and the film boiling regime where the boiling curve is obtained experimentally for a specific fluid. The heat transfer process is modeled using one-dimensional heat conduction with or without heat transfer from the fin tip. Furthermore, five fin profiles are considered: the constant thickness, the trapezoidal, the triangular, the convex parabolic and the parabolic. The multiplicity structure is obtained in order to determine the different types of bifurcation diagrams, which describe the dependence of a state variable of the system (for instance the fin temperature or the heat dissipation) on a design (CCP) or operation parameter (base TD). Specifically the effects of the base TD, of CCP and of the Biot number are analyzed and presented in several diagrams since it is important to know the behavioral features of the heat rejection mechamism such as the number of the possible steady states and the influence of a change in one or more operating variables to these states. Stability analysis is carried out using the “resonance integral” technique and the Sturm-liouville eigen system analysis.

Analysis of Multiplicity Phenomena in Longitudinal Fins Under Multi-Boiling Conditions

2004 ◽  
Vol 126 (1) ◽  
pp. 1-7 ◽  
Author(s):  
Rizos N. Krikkis ◽  
Stratis V. Sotirchos ◽  
Panagiotis Razelos
Keyword(s):  
Heat Transfer ◽  
Temperature Difference ◽  
Solution Structure ◽  
Heat Dissipation ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Constant Thickness ◽  
Base Temperature ◽  
Operating Variables ◽  
Convection Parameter

A numerical bifurcation analysis is carried out in order to determine the solution structure of longitudinal fins subject to multi-boiling heat transfer mode. The thermal analysis can no longer be performed independently of the working fluid since the heat transfer coefficient is temperature dependent and includes the nucleate, the transition and the film boiling regimes where the boiling curve is obtained experimentally for a specific fluid. The heat transfer process is modeled using one-dimensional heat conduction with or without heat transfer from the fin tip. Furthermore, five fin profiles are considered: the constant thickness, the trapezoidal, the triangular, the convex parabolic and the parabolic. The multiplicity structure is obtained in order to determine the different types of bifurcation diagrams, which describe the dependence of a state variable of the system (for instance the fin temperature or the heat dissipation) on a design (Conduction-Convection Parameter) or operation parameter (base Temperature Difference). Specifically the effects of the base Temperature Difference, of the Conduction-Convection Parameter and of the Biot number are analyzed and presented in several diagrams since it is important to know the behavioral features of the heat rejection mechanism such as the number of the possible steady states and the influence of a change in one or more operating variables to these states.

Analysis and Operation Optimization of Recompression Supercritical Carbon Dioxide Power Generation System Based on the Power Flow Method

2018 ◽  
Author(s):  
Xia Li ◽  
Qun Chen ◽  
Xi Chen
Keyword(s):  
Heat Transfer ◽  
Carbon Dioxide ◽  
Power Generation ◽  
Mass Flow ◽  
Power Flow ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Generation System ◽  
Fluid Properties ◽  
Power Generation System

Due to the peculiar physical properties, supercritical carbon dioxide (sCO2) is considered as a promising working fluid in power generation cycles with high reliability, simple structure and great efficiency. Compared with the general thermal systems, the variable properties of sCO2 make the system models obtained by the traditional modelling method more complex. Besides, the pressure distribution in the system will affect the distribution of the fluid properties, the fluid properties influencing the heat transfer process will produce an impact on the temperature distribution which will in turn affect the pressure distribution through the mass flow characteristics of all components. This contribution introduces the entransy-based power flow method to analyze and optimize a recompression sCO2 power generation system under specific boundary conditions. About the heat exchanger, by subdividing the heat transfer area into several segment, the fluid properties in each segment are considered constant. Combining the entransy dissipation thermal resistance of each segment and the energy conservation of each fluid in each segment offers the governing equations for the whole heat transfer process without any intermediate segment temperatures, based on which the power flow diagram of the overall heat transfer process is constructed. Meanwhile, the pressure drops are constrained by the mass flow characteristics of each component, and the inlet and outlet temperatures of compressors and turbines are constrained by the isentropic process constraints and the isentropic efficiencies. Combining the governing equations for the heat exchangers and the constraints for turbine and the compressors, the whole system is modeled by sequential modular method. Based on this newly developed model, applying the genetic algorithm offers the maximum thermal efficiency of the system and the corresponding optimal operating variables, such as the mass flow rate of the working fluid in the cycle, the heat capacity rate of the cold source and the recompression mass fraction under the given heat source. Furthermore, the optimization of the system under different boundary conditions is conducted to study its influence on the optimal mass flow rate of the working fluid, the heat capacity of the cold source and the maximum system thermal efficiency. The results proposes some useful design suggestions to get better performance of the recompression supercritical carbon dioxide power generation system.

Performance Evaluation of a Vertical U-Tube Ground Heat Exchanger Using a Numerical Simulation Approach

2013 ◽  
Vol 724-725 ◽  
pp. 909-915
Author(s):  
Ping Fang Hu ◽  
Zhong Yi Yu ◽  
Fei Lei ◽  
Na Zhu ◽  
Qi Ming Sun ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Performance Evaluation ◽  
Heat Exchanger ◽  
Heat Pump ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Outlet Temperature ◽  
Ground Heat Exchanger ◽  
Ground Source Heat Pump ◽  
Ground Source

A vertical U-tube ground heat exchanger can be utilized to exchange heat with the soil in ground source heat pump systems. The outlet temperature of the working fluid through the U-tube not only accounts for heat transfer capacity of a ground heat exchanger, but also greatly affects the operational efficiency of heat pump units, which is an important characteristic parameter of heat transfer process. It is quantified by defining a thermal effectiveness coefficient. The performance evaluation is performed with a three dimensional numerical model using a finite volume technique. A dynamic simulation was conducted to analyze the thermal effectiveness as a function of soil thermal properties, backfill material properties, separation distance between the two tube legs, borehole depth and flow velocity of the working fluid. The influence of important characteristic parameters on the heat transfer performance of vertical U-tube ground heat exchangers is investigated, which may provide the references for the design of ground source heat pump systems in practice.

Dropwise Condensation on Carbon Steel Surface

2016 ◽  
Author(s):  
Fangyu Cao ◽  
Sean Hoenig ◽  
Chien-hua Chen
Keyword(s):  
Heat Transfer ◽  
Carbon Steel ◽  
Power Plants ◽  
Heat Dissipation ◽  
Low Cost ◽  
Condensation Heat Transfer ◽  
Working Fluid ◽  
High Heat Flux ◽  
Dropwise Condensation ◽  
Two Phase

The increasing demand of heat dissipation in power plants has pushed the limits of current two-phase thermal technologies such as heat pipes and vapor chambers. One of the most obvious areas for thermal improvement is centered on the high heat flux condensers including improved evaporators, thermal interfaces, etc, with low cost materials and surface treatment. Dropwise condensation has shown the ability to increase condensation heat transfer coefficient by an order of magnitude over conventional filmwise condensation. Current dropwise condensation research is focused on Cu and other special metals, the cost of which limits its application in the scale of commercial power plants. Presented here is a general use of self-assembled monolayer coatings to promote dropwise condensation on low-cost steel-based surfaces. Together with inhibitors in the working fluid, the surface of condenser is protected by hydrophobic coating, and the condensation heat transfer is promoted on carbon steel surfaces.

Fin Analysis Under Transition Boiling Heat Transfer: Part II — Radial Fins

2002 ◽  
Author(s):  
Rizos N. Krikkis ◽  
Stratis V. Sotirchos ◽  
Panagiotis Razelos
Keyword(s):  
Heat Transfer ◽  
Power Law ◽  
Biot Number ◽  
Boiling Heat Transfer ◽  
Heat Dissipation ◽  
Thermal Characteristics ◽  
Radius Ratio ◽  
Operating Variables ◽  
Simplifying Assumptions ◽  
Saturated Boiling

The thermal characteristics of six profiles of radial fins subject to transition boiling heat transfer are analyzed. The profiles considered are the rectangular the trapezoidal, the triangular, the convex parabolic, the parabolic and the hyperbolic. The model of the physical mechanism is based on one-dimensional heat conduction using certain simplifying assumptions while the heat transfer coefficient is modeled as a power-law function of the temperature difference between the fin and the saturated boiling liquid with a negative exponent. The problem is formulated by means of dimensionless variables and parameters such as the conduction-convection parameter, the radius ratio and the Biot number that characterize the problem. The multiplicity structure is obtained in order to determine the different types of bifurcation diagrams, which describe the dependence of a state variable of the system (for instance the fin temperature or the heat dissipation) on a design (CCP, radius ratio) or operation parameter (power-law exponent). Specifically the effects of the radius ratio, of the CCP and of the Biot number are analyzed and presented in several diagrams since it is important to know the behavioral features of the heat rejection mechanism such as the number of the possible steady states and the influence of a change in one or more operating variables to these states.

Heat Transfer Augmentation in Solar Collectors Using Nanofluids: A Review

2020 ◽  
Vol 6 (2) ◽  
pp. 72-81 ◽  
Author(s):  
Morteza Anbarsooz ◽  
Maryam Amiri ◽  
Iman Rashidi ◽  
Mohammad Javadi
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Heat Transfer Process ◽  
Heat Transfer Fluid ◽  
Working Fluid ◽  
Optical Parameters ◽  
Solar Collectors ◽  
Direct Absorption ◽  
Heat Transfer Augmentation

Background: Enhancing the heat transfer rate in solar collectors is an essential factor for reducing the size of the system. Yet, various methods have been presented in the literature to increase the heat transfer rate from an absorber to the heat transfer fluid. The most important methods are: the use of evacuated receivers, addition of swirl generators/turbulators and use of various nanofluids as the heat transfer fluid. Objective: The current study reviews the achievements in the enhancement of solar collectors’ heat transfer process using various types of nanofluids. The review revealed that the most widely employed nanoparticles are Al2O3 and Carbon nanotubes (CNTs) and the most popular base fluid is water. Most of the investigations are performed on indirect solar collectors, while recently, the researchers focused on direct absorption methods. In the indirect absorption collectors, the thermal conductivity of the working fluid is essential, while in a direct absorption collector, the optical properties are also crucial. Optimization of the optical parameters along with the thermophysical properties of the nanofluid is suggested for the applications of solar collector.

scholarly journals Nanofluid-Enhancing Shell and Tube Heat Exchanger Effectiveness with Modified Baffle Architecture

2021 ◽  
Author(s):  
I Made Arsana ◽  
Ruri Agung Wahyuono
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Exchanger ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Architectural Geometry ◽  
Tube Heat Exchanger ◽  
Heat Exchanger Design ◽  
Shell And Tube ◽  
Efficient Heat Transfer

As shell and tube heat exchanger is widely employed in various field of industries, heat exchanger design remains a constant optimization challenge to improve its performance. The heat exchanger design includes not only the architectural geometry of either the shell and tube configuration or the additional baffles but also the working fluid. The baffle design including the baffle angle and the baffle distance has been understood as key parameter controlling the overall heat exchanger effectiveness. In addition, a room of improvement is open by substituting the conventional working fluid with the nanomaterials-enriched nanofluid. The nanomaterials, e.g. Al2O3, SiO2, TiO2, increases the thermal conductivity of the working fluids, and hence, the more efficient heat transfer process can be achieved. This chapter provide an insight on the performance improvement of shell and tube heat exchanger by modifying the baffle design and utilizing nanofluids.

scholarly journals Characteristics of Refrigerant Boiling Heat Transfer in Rectangular Mini-Channels during Various Flow Orientations

Energies ◽  
2021 ◽  
Vol 14 (16) ◽  
pp. 4891
Author(s):  
Magdalena Piasecka ◽  
Kinga Strąk
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Channel Length ◽  
Heat Transfer Process ◽  
Heated Wall ◽  
Working Fluid ◽  
Two Phase ◽  
Mini Channels ◽  
The Heat Transfer Coefficient

This paper reports the results of heat transfer during refrigerant flow in rectangular mini-channels at stationary conditions. The impacts of selected parameters on boiling are discussed, i.e., thermal and flow parameters, dimensions and orientation of the channels. Four refrigerants (FC-72, HFE-649, HFE-7000 and HFE-7100) were used as the working fluid. Research was carried out on the experimental set-up with the test section with a single rectangular mini-channel of 180 mm long and with a group of five parallel mini-channels, each 32 mm long. The temperature of the mini-channel’s heated wall was measured by infrared thermography. Local values of the heat transfer coefficient at the contact surface between the fluid and the plate were calculated using the 1D mathematical method. The results are presented as the relationship between the heat transfer coefficient and the distance along the mini-channel length and boiling curves. Two-phase flow patterns are shown. Moreover, the results concerning various refrigerants and the use of modified heater surfaces are discussed. The main factors influencing the heat transfer process were: mini-channel inclination to the horizontal pane (the highest heat transfer coefficient at 270° and 0°), using modified heater surfaces (especially electroerosion texturing and vibration-assisted laser No. 2 texturing) and working fluids (FC-72 and HFE-7000).

The effect of heat loss on the performance of a solar-driven heat engine

2009 ◽  
Vol 223 (7) ◽  
pp. 1615-1621
Author(s):  
O S Sogut ◽  
A Durmayaz
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Heat Loss ◽  
Transfer Process ◽  
Rankine Cycle ◽  
Heat Transfer Process ◽  
Working Fluid ◽  
Fluid Temperature ◽  
Parabolic Trough ◽  
Thermal Reservoir

An optimal performance analysis of a parabolic-trough direct-steam-generation solar-driven Rankine cycle power plant at maximum power (MP) and under maximum power density (MPD) conditions is performed numerically to investigate the effects of heat loss from the heat source and working fluid. In this study, the ideal Rankine cycle of the solar-driven power plant is modified into an equivalent Carnot-like cycle with a finite-rate heat transfer. The main assumptions of this study include that: (a) the parabolic collector is the thermal reservoir at a high temperature, (b) the heat transfer process between the collector and the working fluid is through either radiation and convection simultaneously or radiation only, and (c) the heat transfer process from the working fluid to the low-temperature thermal reservoir is convection dominated. Comprehensive discussions on the effect of heat loss during the heat transfer process from the hot thermal reservoir to the working fluid in the parabolic-trough solar collector are provided. The major results of this study can be summarized as follows: (a) the working fluid temperature at the hot-side heat exchanger decreases remarkably whereas the working fluid temperature at the cold-side heat exchanger does not show any significant change with increasing heat loss, (b) the MP, MPD, and thermal efficiencies decrease with increasing heat loss, and (c) the effect of heat loss on the decrease of thermal efficiency increases when convection is the dominant heat transfer mode at the hot-side heat exchanger.

Orientation-Independent Atomization Heat Transfer Cell for Thermal Management of Microelectronics

2003 ◽  
Author(s):  
Samuel N. Heffington ◽  
Ari Glezer
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Heat Dissipation ◽  
Heated Surface ◽  
Thin Liquid Film ◽  
Transfer Cell ◽  
Working Fluid ◽  
Small Scale ◽  
Operating Characteristics ◽  
Diaphragm Pump

This paper describes a new gravity-independent version of a two-phase cooling, closed heat transfer cell, similar to a thermosyphon. The cooling method is based upon a Vibration-Induced Droplet Atomization, or VIDA, process that can generate small liquid droplets inside a closed cell and propel them onto a heated surface. The VIDA technique involves the violent break-up of a liquid film into a shower of droplets by vibrating a piezoelectric actuator and accelerating the liquid film at resonant conditions. A piezoelectric diaphragm pump is used to supply a constant stream of liquid to the VIDA atomizer enabling gravity-independent operation. The atomized secondary droplets continually coat the heated surface with a thin liquid film that evaporates. The resulting vapor is condensed on internal surfaces of the heat transfer cell as well as the liquid working fluid. The condensed liquid is collected and returned to the atomizing driver by the piezoelectric diaphragm pump. A small-scale gravity independent VIDA atomizer generating spherical droplets of relatively uniform diameter and having sufficient momentum to reach the remotely located heated source has been constructed. Initial test data described in this study include the operating characteristics of the VIDA spray and heat transfer capabilities. Heat dissipation levels as high as 195 W have been measured from an evaporation surface held below 120°C at atmospheric pressure.

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