Increased Heat Transfer Surface Area Research Articles

Endurance Life Estimation of Aircraft Compact Heat Exchanger

Heat Exchangers are used in various engineering systems to transfer heat from a hot fluid to cold fluid across an impermeable wall. The necessity of increased heat transfer surface area has resulted in the development of Compact Heat Exchangers (CHE). Due to their compactness, high performances and less weight, CHE is used for aerospace applications. CHE used in combat aircrafts experience arduous and extreme working conditions during their operation. Before its integration with aircraft, CHEs are required to be tested for performance and qualified for airworthiness by evaluating its endurance life. This paper provides the details of the facility established to experimentally evaluate the endurance life. A computational analysis was carried out to estimate the endurance life with respect to fatigue effects using Finite Element Analysis to calculate stress distribution due to pressure and temperature in CHE during maximum and minimum operating conditions. This was then utilized to calculate fatigue life of CHE. The results were compared with experimental data and it is understood that fatigue alone does not determine the endurance life and there are other parameters that significantly influence the life of CHE.

A review of heat and fluid flow characteristics in microchannel heat sinks

AbstractHeat transfer and flow characteristic in microchannel heat sinks (MCHS) are extensively studied in the literature due to high heat transfer rate capability by increased heat transfer surface area relative to the macroscale heat sinks. However, heat transfer and fluid flow characteristics in MCHS differ from conventional ones because of the scaling effects. This review summarizes the studies that are mainly based on heat transfer and fluid flow characteristic in MCHS. There is no consistency among the published results; however, everyone agrees on that there is no new physical phenomenon in microscale that does not exist at macroscale. Only difference between them is that the effect of some physical phenomena such as viscous dissipation, axial heat conduction, entrance effect, rarefaction, and so forth, is negligibly small at macroscale, whereas it is not at microscale. The effect of these physical phenomena on the heat transfer and flow characteristics becomes significant with respect to specified conditions such as Reynolds number, Peclet number, hydraulic diameter, and heat transfer boundary conditions. Here, the literature was reviewed to document when these physical phenomena become significant and insignificant.

Thermal performance enhancement of energy storage systems via phase change materials utilising an innovative webbed tube heat exchanger

In this study, the phase change materials (PCM) thermal energy storage system utilizing a novel webbed tube heat exchanger was designed and numerically investigated. The obtained simulation results were then compared with three types of conventional heat exchangers. These included: shell and tube heat exchanger, triplex tube heat exchanger and multi-tube heat exchanger. The proposed innovative webbed tube heat exchanger consisted of four horizontal inner heat transfer fluid tubes contained in a shell. The inner heat transfer fluid tubes were connected together by using metal plates welded to these tubes. The additional plates were designed to increase heat transfer surface area. Two-dimensional numerical models were developed. The process of melting (charging) was monitored during the simulation. The acceleration in melting process in the new design was observed owing to improved thermal conductivness.

Performance Optimization of Unglazed Nanofluid Photovoltaic/Thermal System: Energy and Exergy Analyses

The focus of this paper is to predict the transient response of a nanoengineered photovoltaic thermal (PV/T) system in view of energy and exergy analyses. Instead of a circular-shaped receiver, a trapezoidal-shaped receiver is employed to increase heat transfer surface area with photovoltaic (PV) cells for improvement of heat extraction and thus achievement of a higher PV/T system efficiency. The dynamic mathematical model is developed using MATLAB® software by considering real-time heat transfer coefficients. The proposed model is validated with experimental data from a previous study. Negligible discrepancies were found between measured and predicted data. The validated model was further investigated in detail using different nanofluids by dispersing copper oxide (CuO) and aluminum oxide (Al2O3) in pure water. The overall performance of the nanoengineered PV/T system was compared to that of a PV/T system using water only, and optimal operating conditions were determined for maximum useful energy and exergy rates. The results indicated that the CuO/water nanofluid has a notable impact on the energy and exergy efficiencies of the PV/T system compared to that of Al2O3/water nanofluid and water only cases.

Heat flux: thermohydraulic investigation of solar air heaters used in agro-industrial applications

A new design of solar air heater simulator is presented to comply with the extensive applications inagro-industry. A wise installation of increased heat transfer surface area provided uniform and efficient heat diffusion over the duct. Nusselt number and friction factor have been investigated based on the constant roughness parameters such as relative roughness height (e/D), relative roughness pitch (P/e), angle of attack (α) and aspect ratio with Reynolds numbers ranging from 5000 to 19,000 in the fully developed region. Heat fluxes of 800, 900 and 1000 Wm−2 were provided. The enhancement in friction factor is observed to be 3.1656, 3.47 and 3.0856 times, and for the Nusselt number either, augmentation is calculated to be 1.4437, 1.4963 and 1.535 times, respectively, over the smooth duct for 800, 900 and 1000 Wm−2 heat fluxes. Thermohydraulic performance is plotted versus the Reynolds number based on the aforementioned roughness parameters at varying heat fluxes. The results show up that thermohydraulic performance is found to be maximum for 1000 Wm−2 at the average Reynolds number of 5151. Based on the results, we can verify that the introduced solar simulator can help analyzing and developing solar collector installations at the simulated heat fluxes.

Development of Colburn j Factor and Fanning Friction Factor Correlations for Compact Surfaces of the Triangular Perforated Fins Using CFD

The necessity of increased heat transfer surface area has resulted in the development of compact heat exchangers, which are widely used in the aerospace and automobile industries. Hence perforations are made on triangular plain fins to study the effects on the heat transfer coefficient. A numerical model has been developed for the perforated fin of a triangular plate fin heat exchanger. Perforated fin performance has been analyzed with the help of computational fluid dynamics (CFD) by changing the various parameters of the fin. The Colburn j factor and the Fanning friction factor are calculated for different Reynolds numbers. The values of the Colburn j factor and the Fanning friction factor are validated for known geometric fins with available data in the literature and extended to triangular perforated fins. The correlations have been developed between Reynolds number, Colburn j factor, and Fanning friction factor by taking into account fin height, fin thickness, and fin spacing. The present numerical analysis is carried out for air media.

Experimental study of heat transfer enhancement in a liquid piston compressor/expander using porous media inserts

Abstract The efficiency and power density of gas compression and expansion are strongly dependent on heat transfer during the process. Since porous media inserts can significantly increase heat transfer surface area, their addition to a liquid piston compressor/expander has been hypothesized to reduce the time to complete the compression or expansion process and hence the power density for a given thermodynamic efficiency; or to increase the thermodynamic efficiency at a fixed power density. This paper presents an experimental investigation on heat transfer with porous inserts during compression for a pressure ratio of 10 and during expansion for a pressure ratio of 6. A baseline case without inserts and five cases with different porous inserts are tested in a compression experiment: 3 interrupted ABS inserts with plate spacing of 2.5, 5, and 10 mm and 2 aluminum foam inserts sized with 10 and 40 pores per inch. The 2.5 mm and 5 mm interrupted plate inserts were also tested in expansion experiments. Porous inserts are found, in compression, to increase power-density by 39-fold at 95% efficiency and to increase efficiency by 18% at 100 kW / m 3 power density; in expansion, power density is increased three fold at 89% efficiency, and efficiency is increased by 7% at 150 kW / m 3 . Surface area increase is found to be the predominant cause in the improvement in performance. Thus, a liquid piston compressor/expander together with a porous medium may be used in applications requiring high compression ratios, high efficiencies, and high power density such as in an open-accumulator compressed air energy storage (CAES) system or a compressor for compressed natural gas (CNG).

Heat Transfer and Pressure Loss Measurements of Matrix Cooling Geometries for Gas Turbine Airfoils

Matrix cooling systems are relatively unknown among gas turbines manufacturers of the western world. In comparison to conventional turbulated serpentines or pin–fin geometries, a lattice–matrix structure can potentially provide higher heat transfer enhancement levels with similar overall pressure losses. This experimental investigation provides heat transfer distribution and pressure drop of four different lattice–matrix geometries with crossing angle of 45 deg between ribs. The four geometries are characterized by two different values of rib height, which span from a possible application in the midchord region up to the trailing edge region of a gas turbine airfoil. For each rib height, two different configurations have been studied: one having four entry channels and lower rib thickness (open area 84.5%), one having six entry channels and higher rib thickness (open area 53.5%). Experiments were performed varying the Reynolds number Res, based on the inlet subchannel hydraulic diameter, from 2000 to 12,000. Heat transfer coefficients (HTCs) were measured using steady state tests and applying a regional average method; test models have been divided into 20 stainless steel elements in order to have a Biot number similitude with real conditions. Elements are 10 per side, five in the main flow direction, and two in the tangential one. Metal temperature was measured with embedded thermocouples, and 20 thin-foil heaters were used to provide a constant heat flux during each test. A specific data reduction procedure has been developed so as to take into account the fin effectiveness and the increased heat transfer surface area provided by the ribs. Pressure drops were also evaluated measuring pressure along the test models. Uniform streamwise distributions of Nusselt number Nus have been obtained for each Reynolds number. Measurements show that the heat transfer enhancement level Nus/Nu0 decreases with Reynolds but is always higher than 2. Results have been compared with previous literature data on similar geometries and show a good agreement.

Effects of macro and micro roughness in forced convective heat transfer

The article reports a comparative study of macro and micro type artificial roughness in convective heat transfer performance under laminar and low turbulent regime. Circular ribs with different rib height to channel height ratios (e/H=0.05, 0.1, 0.15) fabricated on copper substrate are introduced as macro type roughness whereas copper (Cu) nano-porous layer (avg. thickness about 5μm) is considered as micro roughness. Surface heat transfer and friction characteristics are investigated under different scale roughnesses on one principle wall of a rectangular channel having an aspect ratio (AR) of 7.5. Result shows that the average turbulence intensity between two ribs decreases with decreasing roughness height. On the other hand, nano-porous layer provides significant heat transfer efficiency (about maximum 42% more than bare copper plate) under laminar and low turbulent region without inducing significant turbulence into the channel. Nano-porous layer less than 5μm is found to increase heat transfer surface area significantly that influences the dynamic behaviors of working fluids in the vicinity of heat transfer wall.

Boiling heat transfer enhancement in mini/microtubes via polyhydroxyethylmethacrylate (pHEMA) coatings on inner microtube walls at high mass fluxes

In this experimental study, flow boiling in mini/microtubes was investigated with surface enhancements provided by polyhydroxyethylmethacrylate (pHEMA) coatings (of ∼30 nm thickness) on inner microtube walls. Flow boiling heat transfer experiments were conducted on microtubes (with inner diameters of 249, 507 and 998 µm) having inner surfaces of pHEMA coatings, which increase heat transfer surface area, enable liquid replenishment upon bubble departure, provide additional nucleation sites, and serve for extending critical heat flux (CHF) enhancing boiling heat transfer. The de-ionized water was utilized as the working fluid in this study. pHEMA nanofilms of thickness ∼30 nm on the microtube walls were coated through an initiated chemical vapor deposition technique. Experimental results obtained from the coated microtubes were compared to their plain surface counterparts at two mass flux values (10 000 and 13 000 kg m−2 s−1). In comparison to the plain surface microtubes, the coated surfaces demonstrated an increase up to 24% and 109% in CHF and heat transfer coefficients, respectively. These promising results support the use of pHEMA coated microtubes/channels as a surface enhancement technique for microscale cooling applications.

Submerged jet impingement cooling using nanostructured plates

In this paper, the results of a series of heat transfer experiments conducted on a compact electronics cooling device based on single and two phase jet impingement technique are reported. Deionized and degassed water is propelled into four microchannels. The generated jet impingement is targeted through these channels towards the surface of two nanostructured plates with different surface morphologies placed inside a liquid pool filled with deionized-water. These nanostructured plates are composed of copper nanorods grown on top of a silicon wafer substrate of thickness 350μm coated with a 50nm thick copper thin film layer (i.e. Cu-nanorod/Cu-film/Silicon-wafer). Nanorods were grown using the sputter glancing angle deposition (GLAD) technique. First type of nanostructured plates incorporates ∼600nm long vertically aligned copper nanorod arrays. The second type incorporates ∼600nm long tilted copper nanorod arrays. Heat removal characteristics induced through jet impingement are investigated using the nanostructured plates and compared to the results obtained from a plain surface plate of copper thin film coated on silicon wafer surface. Surface temperatures are recorded by a data acquisition system with four thermocouples integrated on the surface at various prescribed locations. Volumetric flow rate and heat flux values are varied between 107.5–181.5ml/min and 1–40W/cm2. A single phase average heat transfer enhancement of 22.4% and a two phase average heat transfer enhancement of 85.3% has been realized using the nanostructured plate with vertical nanorods compared to flat plate. This enhancement is attributed to the increased heat transfer surface area and the single crystal property of the vertical Cu nanorods. On the other hand, nanostructured plate with tilted nanorods has shown poorer heat transfer performance compared to both the nanostructured plate with vertical nanorods and plain surface plate in the performed experiments. The lower heat transfer rate of the tilted Cu nanorods is believed to be due to the decreased supply of liquid jets to the base of the plate caused by their tilted orientation and closely spaced dense array structure. This leads to formation of air gaps that ultimately become trapped among the tilted nanorods, which results in reduced heat transfer surface area and increased resistance to heat transfer.

Numerical simulation of fluid flow and heat transfer in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall

The paper is focused on the investigation of fluid flow and heat transfer characteristics in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall. In contrast to the new microchannel heat sink, the corresponding conventional rectangular microchannel heat sink is chosen. The computational fluid dynamics is used to simulate the flow and heat transfer in the heat sinks. The steady, laminar flow and heat transfer equations are solved in a finite-volume method. The SIMPLEX method is used for the computations. The effects of flow rate and heat flux on pressure drop and heat transfer are presented. The results indicate that the microchannel heat sink with offset fan-shaped reentrant cavities in sidewall improved heat transfer performance with an acceptable pressure drop. The fluid flow and heat transfer mechanism of the new microchannel heat sink can attribute to the interaction of the increased heat transfer surface area, the redeveloping of the hydraulic and thermal boundary layers, the jet and throttling effects and the slipping over the reentrant cavities. The increased heat transfer surface area and the periodic thermal developing flow are responsible for the significant heat transfer enhancement. The jet and throttling effects enhance heat transfer, simultaneously increasing pressure drop. The slipping over the reentrant cavities reduces pressure drop, but drastically decreases heat transfer.

Effects of structural parameters on fluid flow and heat transfer in a microchannel with aligned fan-shaped reentrant cavities

We provide three-dimensional numerical simulations of conjugate heat transfer in the newly proposed microchannels with different structural parameters. The structural parameters include the lengths and widths of the constant cross-section region and the arcuate region. The effects of structural parameters on pressure drop and thermal resistance are presented. For the first part of the effect analysis, we study the fluid flow and heat transfer mechanism of the microchannel with aligned fan-shaped reentrant cavities in detail, which can attribute to the interaction of the increased heat transfer surface area, the redeveloping boundary layers, the jet and throttling effect and the slipping over the reentrant cavities. As to the second part of the present analysis, the effects of two design structural parameters on fluid flow and heat transfer of the new microchannel are individually prescribed and the two suitable ranges of structural parameter are found for the optimum geometric configuration.

Flow and species transport control in grooved microchannels using local electrokinetic forces

We present numerical simulation results for flow and species transport control in grooved microchannels using locally applied electric fields, and zeta potential patterned groove surfaces. The resulting mixed electro-osmotic/pressure-driven flow enables entrapment and release of prescribed amounts of scalar species in the grooves. The groove size and shape determine the volume of the entrapped species. Depending on the local electric field and zeta potential, each groove can simultaneously contain up to two species. This framework allows control over the interspecies diffusion and mixing using simple flow kinematics, and it is easily applicable for electrically neutral species. We envision utilization of this technique for combinatorial chemistry experiments in a microchannel with multiple grooves, where each groove can be addressed (filled, emptied, or mixed) independently. Alternatively, the technique can be used for electronic cooling, where the grooves increase heat transfer surface area without the adverse effects of recirculating flow pattern.

Thermal performance analysis for solid and perforated blocks attached on a flat surface in duct flow

The thermal performances of solid and perforated rectangular blocks attached on a flat surface in a rectangular duct were determined with respect to the heat transfer from the same plate without blocks. The data used in the performance analyses were obtained experimentally for varying flow and geometrical conditions. It was found that the solid blocks generated a net energy loss despite significantly enhanced heat transfer due to the increased heat transfer surface area. When the blocks were perforated, the loss in the net energy was recovered and depending on the geometrical and flow conditions, a net gain in energy, up to 20%, was achieved. For both the solid and the perforated blocks, increases in Reynolds number led to decreases in the performance.