► Infrared (IR) thermography constitutes a reliable measurement method. ► IR thermography used for the U-Value of building envelopes. ► U-Values obtained validated with measurements and results from ...relevant EN standard. ► Sensitivity analysis is used to find most important parameters influencing accuracy.
Infrared (IR) thermography constitutes a reliable measurement method for the determination of spatially resolved surface temperature distributions. IR thermography may be used for several research problems, applications, and measurement environments with a variety of physical arrangements. In this work the results of the determination of the overall heat transfer coefficient (
U-Value) with the use of IR thermography for building envelopes are presented. The obtained
U-Values are validated by means of measurements performed with the use of a thermohygrometer for two seasons (summer and winter), as well as with the notional results provided by the relevant EN standard. Issues related to the applicability of the method due to the non-steady heat transfer phenomena observed at building shells are also discussed. A more precise validation of the proposed technique was also performed with the use of heat flux meters. The percentage absolute deviation between the notional and the measured
U-Values for IR thermography is found to be in an acceptable level, in the range of 10–20%. Finally, a sensitivity analysis is conducted in order to define the most important parameters which may have a significant influence on the measurement accuracy.
•This review explores passive enhancement of pool boiling using surface modification.•Examined are macrosacle, microscale, nanoscale and multiscale (hybrid-scale) techniques.•Addressed are ...performance goals of inhibiting incidence hysteresis and increasing both nucleate boiling heat transfer coefficient and CHF.•Shown is that micro and nano surface features are susceptible to blockage and changes in performance over time.
This paper provides a comprehensive review of published articles addressing passive enhancement of pool boiling using surface modification techniques. They include macroscale, microscale, and nanoscale surfaces, as well as multiscale (hybrid-scale), and hybrid-wettability techniques. Different enhancement methods are assessed in terms of underlying fluid routing mechanisms and ability to achieve three distinct heat transfer goals: eliminating incipient boiling hysteresis, increasing nucleate boiling heat transfer coefficient, and ameliorating critical heat flux (CHF), especially for inert dielectric coolants that are both highly wetting and possess relatively poor thermophysical properties. While different enhancement scales are shown to provide different degrees of success in achieving the three goals, it is shown that both microscale and nanoscale surface features are susceptible to blockage, resulting in deterioration of the enhancement over time. This review also points to scarcity of sufficiently sized databases for a given enhancement scheme in terms of fluid type, surface material, size, and orientation, enhancement shape, pattern, and scale, and operating pressure. This renders available findings less-than-adequate tools for design of practical cooling systems.
CO2 utilization is an evolving technology that converts CO2 into CH4 or other chemicals upon its reaction with H2, which can be supplied from water electrolysis using the surplus solar and wind ...power. CO2 methanation and hydrogenation reactions require thermal management as they are exothermic. The average heat transfer coefficient (ho) was measured and analyzed at various temperatures (200°C–400°C), gas velocities (0.7–9.1 Uo/Umf), and pressures (1–25 bar) using two different fluidized-bed setups for CO2 methanation and hydrogenation reactions. In addition, the effects of the heat exchanger location at different heights in a fluidized-bed reactor were studied along with the local heat transfer coefficients (hL) of the tube. The glass bead resulted in higher ho (200–340 W/m2∙°C) compared to the nickel-based material (133–183 W/m2∙°C) owing to its bubble formation characteristics. An increase in gas velocity, temperature, and pressure enhanced the heat transfer efficiency. The reason for the different ho values at different heights was inferred from the local heat transfer coefficients (hL) of the tube. The highest heat transfer rate was observed at the bottom of the tube, and the lowest heat transfer rate was found to be on either side or at the top of the tube depending on the location of the heat exchanger in the fluidized-bed reactor. Finally, an empirical equation was derived for ho under CO2 hydrogenation conditions, showing a p-value of <0.0001. Our study can pave the way for highly effective heat exchanger design for CO2 utilization.
Heat transfer coefficients were obtained for CO2 hydrogenation and methanationGas velocity and temp. affected the average heat transfer coefficient (133–340 W/m2∙°C)Local and average heat transfer coefficients were analyzed to investigate the difference in hoEffects of high-pressure (up to 25 barg) were investigated for CO2 hydrogenation conditions
This second part of a two-part study examines the prediction of saturated flow boiling heat transfer in mini/micro-channels. The first part culminated in a technique for determining the dryout ...incipience quality corresponding to substantial deterioration in the heat transfer coefficient. In this part, a consolidated database for flow boiling in mini/micro-channels is amassed from 31 sources, of which 10,805 data points are designated as pre-dryout. The pre-dryout database consists of 18 working fluids, hydraulic diameters of 0.19–6.5mm, mass velocities of 19–1608kg/m2s, liquid-only Reynolds numbers of 57–49,820, qualities of 0–1, and reduced pressures of 0.005–0.69. The pre-dryout database is used to evaluate prior correlations that have been recommended for both macro-channels and mini/micro-channels. A few of these correlations are shown to provide fair overall performance, but their accuracy is compromised against specific portions of the database, especially high pressures and very small diameters. A new generalized correlation is constructed by superpositioning the contributions of nucleate boiling and convective boiling. This correlation is shown to provide very good predictions against the entire pre-dryout database, evidenced by an overall MAE of 20.3%, with 79.9% and 95.5% of the data falling within ±30% and ±50% error bands, respectively. Evenly good predictions are achieved for all working fluids and all ranges of the database parameters.
•This review explores effects of additives and nanofluids on nucleate boiling heat transfer coefficient and critical heat flux (CHF).•Surfactants are shown to enhance nucleate boiling, but ...enhancement potential of polymers is polymer specific.•Nanofluids enhance CHF significantly, but may have negative impact on nucleate boiling heat transfer coefficient.•Serious practical concerns in deploying nanofluids in cooling applications are discussed.
Enhancement of nucleate pool boiling by modifying fluid properties has drawn considerable attention in recent years. This paper provides a comprehensive review of published literature concerning enhancement methodologies of surfactant and polymer additives, and nanofluids. Each method is discussed in detail in terms of measured impact on the nucleate boiling heat transfer coefficient and critical heat flux (CHF), mechanisms proposed for any heat transfer enhancement, and predictive models. It is shown that adding surfactant to base liquid shifts the nucleate boiling region of the boiling curve towards lower surface superheats, thereby promoting earlier boiling incipience and increasing the nucleate boiling heat transfer coefficient, but the heat transfer merits of polymer addition are polymer specific. Despite significant enhancement in CHF with most nanofluids, there are many contradictory findings concerning influence of nanofluids on nucleate boiling heat transfer coefficient. These contradictions are the result of many complex influences of base liquid, nanoparticles, and initial surface roughness. Despite the potential heat transfer benefits of nanofluids, there are several serious practical concerns that must be considered carefully before deploying nanofluids in practical cooling applications.
•This review explores potential convective heat transfer merits of nanofluids.•Both experimental and numerical findings are reviewed, including macro- and microchannels.•It is shown that heat ...transfer with nanofluids is realized mostly in inlet single-phase region.•Serious practical concerns in deploying nanofluids in cooling situations are identified.
This paper provides a comprehensive review of published literature concerning heat transfer benefits of nanofluids for both macro-channels and micro-channels. Included are both experimental and numerical findings concerning several important performance parameters, including single-phase and two-phase heat transfer coefficients, pressure drop, and critical heat flux (CHF), each being evaluated based on postulated mechanisms responsible for any performance enhancement or deterioration. The study also addresses issues important to heat transfer performance, including entropy minimization, hybrid enhancement methodologies, and nanofluid stability, as well as the roles of Brownian diffusion and thermophoresis. Published results point to appreciable enhancement in single-phase heat transfer coefficient realized in entrance region, but the enhancement subsides downstream. And, while some point to the ability of nanofluids to increase CHF, they also emphasize that this increase is limited to short duration boiling tests. Overall, studies point to many important practical problems associated with implementation of nanofluids in cooling situations, including clustering, sedimentation, and precipitation of nanoparticles, clogging of flow passages, erosion to heating surface, transient heat transfer behavior, high cost and production difficulties, lack of quality assurance, and loss of nanofluid stability above a threshold temperature.
•Effect of channel angle on thermal performance of pin fin heat sink is reported.•Water based graphene nanoplatelets (GNPs) nanofluids are used as test fluid.•HTC for 22.5 degree heat sink is 84.30% ...higher as compared 90 degree heat sink.•Nanofluids compared to distilled water showed about 20% heat transfer enhancement.
This study reports an experimental work to examine the angle effect of pin fin heat sink channel in terms of convective heat transfer coefficient, log mean temperature difference and thermal resistance using water based graphene nanoplatelets (GNPs) nanofluids in a flow rate range of 0.25–0.75 LPM. Three heat sinks having channel angles, measured from positive x-axis, 22.5 degree, 45 degree and 90 degree are used. The volumetric concentration of GNPs particles is 9.5% and these particles consist of overlapped two-dimensional graphene layers. All heat sinks are fabricated with copper substrate, which is maintained at uniform heat flux during experimentation. Heat sink with 22.5 degree channel angle shows better thermal performance as compared to other tested heat sinks. For the same flow rate, 22.5 degree heat sink shows lowest convective thermal resistance as compared to other tested heat sinks.
Boiling heat transfer has drawn continuous attention owing to its wide range of applications in the energy fields. In the boiling process, the heat transfer coefficient (HTC) enhancement is needed ...for better energy conversion efficiency, and the critical heat flux (CHF) enhancement is needed to avoid boiling crises. With the advancement of nanotechnology, nanoscale surface modifications and nanofluids have shown great boiling enhancement potential. Furthermore, the coupled methods, which refer to the adoption of more than one method for boiling heat transfer enhancement, provide a novel way for possibly enhancing the HTC and CHF simultaneously. This work aims to provide the latest review of boiling enhancement using nanotechnology and its coupled methods. The nanotechnology-based methods on boiling enhancement including nanoscale modified surfaces and nanofluids are summarized. Furthermore, different coupled methods, including (a) hybrid nanofluids; (b) combined methods of using both nanofluid and surface modification; (c) bi-feature surfaces with multi-wettability (biphilic), multi-material, or multi-scale coating for surface modification, are discussed. Biphilic surfaces refer to surfaces with both hydrophilic and hydrophobic regions. Multi-materials surfaces refer to surfaces adopting two coating materials in the fabrication process, and multi-scale surfaces refer to nano/micro-scale surfaces in this review. The CHF and the HTC enhancement of pool and flow boiling are discussed, and conclusions and recommendations for future work are presented.
This article provides a comprehensive review of published literature concerning enhancement of channel flow boiling heat transfer by surface modification. Addressed are macro, micro, nano, and hybrid ...multiscale methodologies. While the vast majority of published schemes have shown favorable heat transfer performance, evidenced by earlier onset of boiling, improved flow boiling heat transfer coefficient, and ameliorated critical heat flux (CHF), increased pressure drop is a serious concern shared by most. Nanoscale enhancement remains controversial, given that, unlike macroscale and microscale enhancement features, nanostructure topographies are prone to severe degradation after prolonged boiling. Multiscale enhancement, combining macro, micro, and nano features as well as favorable coolant flow control, is shown highly effective at tackling very high heat flux situations. Also included in this review is discussion of effective means for mitigating flow instabilities. It is concluded that the most serious obstacle to adopting surface modification techniques is absence of generalized predictive tools or robust computational models for different channel shapes, sizes, and enhancement configurations, and different fluids and operating conditions, such as tools presently available for two-phase flow and heat transfer in conventional micro-channel heat sinks.
Porous structures are ubiquitous in many thermal management and energy conversion systems. The morphology of a porous structure has significant impact on the fluid flow, heat/mass transport, and ...strength performance. However, the available fabrication techniques are not capable of directly tailoring porous structures with well-controlled pore features and functional graded pore morphology, thereby limiting the performance enhancements for these systems. In this study, a triply periodic minimal surface (TPMS) based method was developed to customize the morphology of porous media with well-controlled pore features and to fabricate these parts with additive manufacturing. Porous structures with designed pore parameters were built based on the mathematically defined iWP surface, primitive surface, diamond surface and gyroid surface. Before performing flow and heat transfer simulations, morphological analysis was conducted to establish the connections between the geometrical parameters and the performance of the porous structure (flow resistance, heat transfer, and strength). The porous structures were compared in terms of their structural strength, specific pressure drop, interstitial/volumetric heat transfer coefficients and the ratio of the Colburn factor relative to the friction factor. The TPMS porous structures indicated that much higher strength than the simple cubic packing structure (approximation of sintered metal particles) due to their reasonable struct connectivity. Computations demonstrated that the type P structure had the lowest flow resistance and highest comprehensive heat transfer coefficient (j/f). The high specific surface area, continuous changes in the flow direction, periodic mixing/redistribution and flow acceleration contributed to the higher volumetric heat transfer coefficients in the type W/G structures.
