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6 pages, 190 KiB

Open AccessEditorial

Heat and Mass Transfer Issues in Mini-Gaps

by Magdalena Piasecka

Energies 2023, 16(16), 5854; https://doi.org/10.3390/en16165854 - 8 Aug 2023

Viewed by 643

Abstract

The transfer of large heat fluxes is one of the most significant issues in modern technology [...] Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

Research

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0 pages, 5618 KiB

Open AccessArticle

Pool Boiling of Novec-649 on Inclined Microchannel

by Robert Kaniowski

Energies 2023, 16(5), 2476; https://doi.org/10.3390/en16052476 - 5 Mar 2023

Cited by 4 | Viewed by 1671

Abstract

Significant amounts of heat flow can be removed with small temperature differences by boiling. This process is used in a variety of industries, e.g., cooling electronics, digital circuits, power sources, etc. Heat dissipation from equipment that generates significant heat flows involves the movement [...] Read more.

Significant amounts of heat flow can be removed with small temperature differences by boiling. This process is used in a variety of industries, e.g., cooling electronics, digital circuits, power sources, etc. Heat dissipation from equipment that generates significant heat flows involves the movement of thermal energy through a wall into a cooling fluid. In the present study, this mechanism was analysed during the boiling of Novec-649 fluid at atmospheric pressure. The heat transfer surfaces were samples made of copper with milled, parallel grooves with a depth of 0.3 mm and a width ranging from 0.2 to 0.5 mm in 0.1 mm increments for straight channels and channels inclined with respect to the vertical by 30° and 60°, respectively. The study was carried out from the onset of nucleate boiling, approximately q = 7 kWm⁻² with a heat flux increase until the critical heat flux was reached. The maximum heat flux was 262 kWm⁻² and the heat transfer coefficient was 19.4 kWm⁻²K⁻¹, achieved for surfaces with straight microchannels. A maximum heat flux increased by 80% and a heat transfer coefficient twice as high compared to a smooth surface was obtained. The performance of the experiment can be deemed adequate, considering that it compares well with the correlation results of different authors. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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26 pages, 7155 KiB

Open AccessFeature PaperArticle

Boiling Heat Transfer during Flow in Vertical Mini-Channels with a Modified Heated Surface

by Magdalena Piasecka and Kinga Strąk

Energies 2022, 15(19), 7050; https://doi.org/10.3390/en15197050 - 26 Sep 2022

Cited by 5 | Viewed by 1332

Abstract

The process with change of phase during flow in mini-channels plays a significant role in many industrial applications, such as microelectronics. Furthermore, methods for heat transfer intensification during flow in channels of small cross-section are still being sought. In this work, studies of [...] Read more.

The process with change of phase during flow in mini-channels plays a significant role in many industrial applications, such as microelectronics. Furthermore, methods for heat transfer intensification during flow in channels of small cross-section are still being sought. In this work, studies of the effect of using a modified heated surface on intensification boiling heat transfer in rectangular mini-channels during upward and downward flow are performed. The test section of a group of seven parallel mini-channels 1 mm deep was investigated during the subcooled and saturated flow boiling of FC-72. The temperature of the outer heated wall surface was measured using an infrared camera. During the experiments, two-phase flow structures were captured with a quick camera. Local heat transfer coefficients at the contact surface between the working fluid and the heated surface were determined with the use of a one-dimensional calculation method. To present the results, local temperature measurements and heat transfer coefficients, boiling curves and two-phase flow patterns are shown and analyzed. The results concerning two directions of vertical flow along mini-channels are discussed. Several modified heated surfaces and one smooth were tested for comparison. The main objective was to find out how the modified surface of the heated wall can intensify boiling heat transfer with upward and downward refrigerant flow in mini-channels of rectangular cross-section. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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21 pages, 8749 KiB

Open AccessArticle

Pool Boiling of Water on Surfaces with Open Microchannels

by Robert Kaniowski and Robert Pastuszko

Energies 2021, 14(11), 3062; https://doi.org/10.3390/en14113062 - 25 May 2021

Cited by 25 | Viewed by 3107

Abstract

Boiling, as the most efficient type of convective heat transfer, is an area of interest in many fields of industry and science. Many works have focused on improving the heat transfer efficiency of boiling by altering the physical and chemical properties of surfaces [...] Read more.

Boiling, as the most efficient type of convective heat transfer, is an area of interest in many fields of industry and science. Many works have focused on improving the heat transfer efficiency of boiling by altering the physical and chemical properties of surfaces by using different technological processes in their fabrication. This paper presents experimental investigations into pool boiling on enhanced surfaces with open microchannels. The material of the fabricated surface was copper. Parallel microchannels made by machining were about 0.2, 0.3, and 0.4 mm wide, 0.2 to 0.5 mm deep, and spaced with a pitch equal to twice the width of the microchannel. The experiments were carried out in water at atmospheric pressure. The experimental results obtained showed an increase in the heat flux and the heat transfer coefficient for surfaces with microchannels. The maximum (critical) heat flux was 2188 kW/m², and the heat transfer coefficient was 392 kW/m²K. An improvement in the maximum heat flux of more than 245% and 2.5–4.9 times higher heat transfer coefficient was obtained for the heat flux range of 992–2188 kW/m² compared to the smooth surface. Bubble formation and growth cycle in the microchannel were presented. Two static computational models were proposed to determine the bubble departure diameter. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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23 pages, 8403 KiB

Open AccessArticle

Experimental Study on Thermal Performance of a Loop Heat Pipe with Different Working Wick Materials

by Kyaw Zin Htoo, Phuoc Hien Huynh, Keishi Kariya and Akio Miyara

Energies 2021, 14(9), 2453; https://doi.org/10.3390/en14092453 - 25 Apr 2021

Cited by 3 | Viewed by 2264

Abstract

In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) [...] Read more.

In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m⁻²). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m⁻²). Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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23 pages, 7710 KiB

Open AccessArticle

Curved Surface Minijet Impingement Phenomena Analysed with ζ-f Turbulence Model

by Tomasz Kura, Elzbieta Fornalik-Wajs, Jan Wajs and Sasa Kenjeres

Energies 2021, 14(7), 1846; https://doi.org/10.3390/en14071846 - 26 Mar 2021

Cited by 6 | Viewed by 1746

Abstract

The jet impingement phenomenon plays an important role among the heat transfer intensification methods. Very often its application and analyses refer to simple flat surfaces, while there is a lack of information in the literature for cases addressing curved surfaces. In the present [...] Read more.

The jet impingement phenomenon plays an important role among the heat transfer intensification methods. Very often its application and analyses refer to simple flat surfaces, while there is a lack of information in the literature for cases addressing curved surfaces. In the present work, the single jet impingement on the non-flat (concave and convex) surface is studied for a wide range of geometries, which originate from the mini-jet heat-exchanger design. The numerical simulations were performed by an advanced ζ-f turbulence model implemented in the open-source OpenFOAM (ESI-OpenCFD Ltd, Bracknell, United Kingdom) code. Noticeable differences in the phenomena occurring on the convex and concave surfaces were identified in the stagnation zone. Besides, the existence and location of the secondary peak in the Nusselt number distribution differed between the cases. These distributions were influenced by the shape of geometry, which determined flow characteristics and resulting heat transfer performance. The origins of these differences were looked at in the turbulence characteristics close to the impinged surface of the stagnations zone and its vicinity, where turbulence kinetic energy and enstrophy were analysed. It was stated that the differences are already noticeable for the single jet impingement case, but they might sum up when multiple jets are considered. Therefore, here presented results would be important for the analysis of the overall unit of mentioned mini-jets heat-exchanger. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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24 pages, 6619 KiB

Open AccessArticle

Experimentally Verified Flow Distribution Model for a Composite Modelling System

by Dominika Babička Fialová and Zdeněk Jegla

Energies 2021, 14(6), 1778; https://doi.org/10.3390/en14061778 - 23 Mar 2021

Cited by 3 | Viewed by 2066

Abstract

Requirements of modern process and power technologies for compact and highly efficient equipment for transferring large heat fluxes lead to designing these apparatuses as dense parallel flow systems, ranging from conventional to minichannel dimensions according to the specific industrial application. To avoid operating [...] Read more.

Requirements of modern process and power technologies for compact and highly efficient equipment for transferring large heat fluxes lead to designing these apparatuses as dense parallel flow systems, ranging from conventional to minichannel dimensions according to the specific industrial application. To avoid operating issues in such complex equipment, it is vital to identify not only the local distribution of heat flux in individual parts of the heat transfer surface but also the uniformity of fluid flow distribution inside individual parallel channels of the flow system. A composite modelling system is currently being developed for accurate design of such complex heat transfer equipment. The modeling approach requires a flow distribution model enabling to yield accurate-enough predictions in reasonable time frames. The paper presents the results of complex experimental and modeling investigation of fluid flow distribution in dividing headers of tubular-type equipment. Different modeling approaches were examined on a set of header geometries. Predictions obtained via analytical and numerical models were validated using data from the experiments conducted on additively manufactured header samples. Two case studies employing parallel flow systems (mini-scale systems and a conventional-size heat exchanger) demonstrated the applicability of the distribution model and the accuracy of the composite modelling system. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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20 pages, 5087 KiB

Open AccessArticle

Thermal and Hydrodynamic Phenomena in the Stagnation Zone—Impact of the Inlet Turbulence Characteristics on the Numerical Analyses

by Tomasz Kura, Jan Wajs, Elzbieta Fornalik-Wajs, Sasa Kenjeres and Sebastian Gurgul

Energies 2021, 14(1), 105; https://doi.org/10.3390/en14010105 - 28 Dec 2020

Cited by 5 | Viewed by 1795

Abstract

One of the methods of heat transfer enhancement is utilization of the turbulent impinging jets, which were recently applied, for example, in the heat exchangers. Their positive impact on the heat transfer performance was proven, but many questions related to the origin of [...] Read more.

One of the methods of heat transfer enhancement is utilization of the turbulent impinging jets, which were recently applied, for example, in the heat exchangers. Their positive impact on the heat transfer performance was proven, but many questions related to the origin of this impact are still unanswered. In general, the wall-jet interaction and the near-wall turbulence are supposed to be its main reason, but their accurate numerical analysis is still very challenging. The authors’ aim was to construct the numerical model which can represent the real phenomena with good or very good accuracy. Starting with an analysis of single jet and obtaining the agreement with experimental data, it will be possible to extend the model towards the whole minijets heat exchanger. The OpenFOAM software, Bracknell, UK was used for that purpose, with our own implementation of the ζ-f turbulence model. The most difficult area to model is the stagnation region, where the thermal effects are the most intensive and, at the same time, strongly affected by the conditions in the pipe/nozzle/orifice of various size (conventional, mini, micro), from which the jet is injected. In the following article, summary of authors’ findings, regarding significance of the velocity profile and turbulence intensity at the jet place of discharge are presented. In addition, qualitative analysis of the heat transfer enhancement is included, in relation to the inlet conditions. In the stagnation point, Nusselt number differences reached the 10%, while, in general, its discrepancy in relation to inlet conditions was up to 23%. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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25 pages, 8736 KiB

Open AccessArticle

Heat Transfer Coefficient Determination during FC-72 Flow in a Minichannel Heat Sink Using the Trefftz Functions and ADINA Software

by Magdalena Piasecka, Beata Maciejewska and Paweł Łabędzki

Energies 2020, 13(24), 6647; https://doi.org/10.3390/en13246647 - 16 Dec 2020

Cited by 14 | Viewed by 2012

Abstract

This work focuses on subcooled boiling heat transfer during flow in a minichannel heat sink with three or five minichannels of 1 mm depth. The heated element for FC-72 flowing along the minichannels was a thin foil of which temperature on the outer [...] Read more.

This work focuses on subcooled boiling heat transfer during flow in a minichannel heat sink with three or five minichannels of 1 mm depth. The heated element for FC-72 flowing along the minichannels was a thin foil of which temperature on the outer surface was measured due to the infrared thermography. The test section was oriented vertically or horizontally. A steady state heat transfer process and a laminar, incompressible flow of the fluid in a central minichannel were assumed. The heat transfer problem was described by the energy equations with an appropriate system of boundary conditions. Several mathematical methods were applied to solve the heat transfer problem with the Robin condition to determine the local heat transfer coefficients at the fluid/heated foil interface. Besides the 1D approach as a simple analytical method, a more sophisticated 2D approach was proposed with solutions by the Trefftz functions and ADINA software. Finite element method (FEM) calculations were conducted to find the temperature field in the flowing fluid and in the heated wall. The results were illustrated by graphs of local heated foil temperature and transfer coefficients as a function of the distance from the minichannel inlet. Temperature distributions in the heater and the fluid obtained from the FEM computations carried out by ADINA software were also shown. Similar values of the heat transfer coefficient were obtained in both the FEM calculations and the 1D approach. Example boiling curves indicating nucleation hysteresis are shown and discussed. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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13 pages, 6140 KiB

Open AccessArticle

Experimental and Numerical Analysis of the Gas Flow in the Axisymmetric Radial Clearance

by Damian Joachimiak and Andrzej Frąckowiak

Energies 2020, 13(21), 5794; https://doi.org/10.3390/en13215794 - 5 Nov 2020

Cited by 11 | Viewed by 1448

Abstract

This paper focuses on the analysis of the gas flow in the axisymmetric mini gap bounded by the surface of the top of the labyrinth seal tooth and the surface of the body. It includes the results of experimental research and numerical calculations. [...] Read more.

This paper focuses on the analysis of the gas flow in the axisymmetric mini gap bounded by the surface of the top of the labyrinth seal tooth and the surface of the body. It includes the results of experimental research and numerical calculations. Experimental research focused on the analysis of gas flow for six clearance heights in a wide range of pressure drops. Based on this research, we determined the mass flow in the clearance. Using the Saint-Venant equation, we determined the flow coefficient versus the pressure ratio upstream and downstream from the seal. Flow coefficients for various clearance heights obtained from the experiment can be divided into two data groups, the values of which differ significantly. To explain changes in the value of the gas flow coefficient for selected clearance heights, numerical analysis of the said gas flow was performed using the Fluent software. This analysis allowed us to explain the reason for the variability of the flow coefficient. This research can be the basis for determining the change of seal integrity during operation for staggered and stepped seals. Full article

(This article belongs to the Special Issue Heat and Mass Transfer Issues in Mini Gaps)

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