Experimental Investigation Of Heat Transfer Research Articles

Experimental Investigations of Heat Transfer in Simultaneously Developing Transitional Regime of Mixed Convection Flows in a Vertical Tube

Abstract The study of flow behaviour in the simultaneously developing transitional regime of mixed convection flows is rare. It has been believed that the transitional regime will give a good compromise between pressure drop and heat transfer compared to laminar and turbulent flow regime. In this experimental study, the friction factor (f) and Nusselt number (Nu) characteristics for buoyancy-assisted and opposed flows of water in concurrently developing transitional regime of mixed convection through a vertical tube are studied. Experiments were done for Reynolds numbers (Re) varying from 500 to 15000, Grashof numbers (Gr) from 1.25×104 to 5×106, Prandtl numbers (Pr) from 3 to 7, and Richardson numbers (Ri) from 0 to 0.1 subjected to uniform heat flux boundary conditions. A flow visualization provision after the test section which confirms an early transition in buoyancy-opposing flow (Rec = 2264) compared to buoyancy-aiding flow (Rec = 2468) at a fixed Ri of 0.1. Further, with the increase in Ri from 0 to 0.1, the average f decreases, and the average Nu increases in both aiding and opposing flows. It confirms that, the onset of transition gets delayed with the increase of heat flux supplied in both the flows. Based on the present outcomes, an efficient heat exchanging device can be operated either to delay or advance the transition in a vertical pipe flow for optimum heat transfer.

Experimental investigation of heat transfer and aerodynamic drag of novel heat sinks with lamellar fins

Heat transfer and aerodynamic drag of novel small-sized heat sinks with lamellar fins, designed for electronic cooling, were experimentally investigated under conditions of forced convection in the range of Reynolds numbers 1 250–10 500. It was found that a gradual reduction in the fin spacing from 6 mm to 3 mm with a 29° angle of taper between the outermost fins leads to an increase in the heat transfer intensity by 15–32% with a significant increase in aerodynamic drag compared to the surface with a constant fin spacing of 6 mm. Incomplete cross-section cutting of fins at the relative depth of 0.6 in addition to the gradual reduction in the fin spacing provides aerodynamic drag decrease by 5–20% and increase of heat transfer intensity by 18–20% in comparison with the similar heat sink without fins cutting. Proposed novel designs of heat sinks enabled us to decrease by 7°С–16°С the maximum overheating of the heat sink's base in the flow speed range from 2.5 m/s to 7.5 m/s at constant heat load. To ensure a constant value of maximum overheating of the heat sink base the inlet flow velocity for the surface with constant fin spacing should be 1.6–2 times higher than that for the heat sink with 29° taper angle between outermost fins and partially fins cutting. In this case, the aerodynamic drag for the latter will be higher only by 1.6–2.7 times, which is quite acceptable.

Experimental Investigation of Heat Transfer and Pressure Drop Performance of a Circular Tube with Coiled Wire Inserts

This paper evaluates the thermo-hydraulic performance of a coiled wire passive insert for internal turbulent flow through a circular copper tube test section in an in-tube exchanger. Experiments were carried out using water as the working fluid with Reynolds number ranging from 8000 to 32000. The experimental setup was validated for Nusselt number and friction factor with well-established equations for plain tubes. The average Nusselt number ratios (Nua/Nup) and the friction factor ratios (fa/fp) for the augmented tube case over the plain tube case are reported to range from 1.55 to 1.38 and from 1.513 to 1.583, respectively. The average performance ratios considering equal pumping power criteria are also reported and found in the range of 0.846 to 0.921. The study concludes that coiled wire inserts are suitable for heat transfer augmentation applications where pumping power is of minor concern.

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS OF FERROFLUIDS IN THE PRESENCE OF MAGNETIC FIELD AND LAMINAR FLOW CONDITIONS

In this study, the heat transfer performance with forced convection of two different water-based nanofluids was investigated by applying an alternating magnetic field in a minichannel. CoFe<sub>2</sub>O<sub>4</sub>-water and MnFe<sub>2</sub>O<sub>4</sub>-water nanofluids have been prepared at 0.5 vol.% and tested. The tests were carried out in a minichannel under laminar flow conditions in the Reynolds numbers range of 300-1700. Nusselt numbers of each fluid used in the experiments were calculated and compared. At the Reynolds number of 1500, the CoFe<sub>2</sub>O<sub>4</sub>-water nanofluid exhibited an increase of 12% compared to pure water, while the MnFe<sub>2</sub>O<sub>4</sub>-water nanofluid showed an increase of 4%. The Nusselt number increased in both nanofluids by applying the magnetic field to nanofluids. The highest Nusselt number obtained was 9.35 for the CoFe<sub>2</sub>O<sub>4</sub>-water nanofluid in the presence of magnetic field. While this increase was more pronounced at low Reynolds numbers, a lower rate of increase was obtained at high Reynolds numbers. In addition, the use of nanofluids significantly increased the pressure drop compared to the base fluid. While an almost 100% increase in the pressure drop was observed for the CoFe<sub>2</sub>O<sub>4</sub>-water nanofluid compared to pure water, the 65% increase for the MnFe<sub>2</sub>O<sub>4</sub>-water nanofluid was maximum. At the highest Reynolds numbers, the maximum pressure drops were determined as 3.4 kPa for the CoFe<sub>2</sub>O<sub>4</sub>-water nanofluid and 3 kPa for the MnFe<sub>2</sub>O<sub>4</sub>-water nanofluid. It was also detected that the friction factor for CoFe<sub>2</sub>O<sub>4</sub>-water and MnFe<sub>2</sub>O<sub>4</sub>-water nanofluids was 80% and 40% higher, respectively, than for the base fluid.

Experimental investigation of heat transfer to carbon dioxide in parallel rectangular channels under supersonic mainstream

Active cooling using carbon dioxide(CO2) is a promising method for supersonic vehicles' combustion chamber, but few studies have considered CO2 heat transfer under supersonic mainstream conditions. In this paper, the double-wall heat transfer effect of CO2 under supersonic mainstream conditions is investigated by using the direct-connection experimental platform. The double-wall consists of an inner liner and a cooling jacket with parallel rectangular channels. The inlet CO2 parameter conditions range for T (Temperature) = 263.3 ∼ 272.5 K, m (Mass flow rate) = 50 ∼ 205.3 g/s, and P (Pressure) = 3.87 ∼ 8.88 MPa. It has been found that the magnitude of CO2 mass flow rate has a very significant effect on the effectiveness of thermal protection. The CO2 active cooling provides effective thermal protection for the inner liner at inlet parameter m = 199.1 ∼ 205.3 g/s, P = 8.69 ∼ 8.88 MPa, and T = 263.3 ∼ 266.8 K. Moreover, the heat transfer reaches thermal equilibrium quickly. However, it is more effective in thermally protecting the inner liner near the CO2 inlet than near the CO2 outlet at the inlet parameters of m = 50 ∼ 88.6 g/s, P = 3.87 ∼ 7.74 MPa, and T = 263.3 ∼ 272.5 K. In addition, there may be a risk of structural burnout near the CO2 outlet.

Experimental investigation of heat transfer and structure optimization for regenerative cooling channels using n-decane

Regenerative cooling technologies are considered one of the most effective and widely used active thermal protection methods for hypersonic aircraft scramjets. The heat transfer performance of hydrocarbon fuel and the geometric optimization of cooling channels are the key factors to be considered in designing regenerative cooling structures. Therefore, the present study experimentally and numerically investigated the heat transfer characteristics of n-decane in regenerative channels under asymmetric heating conditions and presented a new structure optimization method. The experiment was conducted at 3 MPa and 293 K with a mass flow rate of 0∼2.442 g/s per channel and a heat flux range of 0∼150 kW/m2. Experimental data show that the local Nusselt numbers exhibit a higher sensitivity to changes in mass flow rate compared to heat flux. Compared to symmetric heating, asymmetric heating mainly affects the heat conduction in the solid domain rather than the performance of convective heat transfer. The modified Dittus-Boelter correlation could predict the experimental data with a relative deviation of ±25 %. A rapid temperature prediction method based on heat flux distribution was proposed through heat transfer analysis, and it was utilized with a particle swarm optimization algorithm to conduct structure optimization.

Experimental investigation of heat transfer in the trapezoidal channel with lateral outflow under rotation conditions

The heat transfer characteristics of wedge-shaped transverse outflow channels with different pin-fins under rotating conditions were measured experimentally. The Reynolds number varies from 10,000 to 70,000, and the rotation number varies from 0 to 0.6. For non-rotating state, the Nussel number decrease along the flow direction on the pin–fin rough surface as inner smooth area, albeit marginally. For rotating state, the enhancement of Nusselt number ratio in the outer pin-fins area is significantly weaker than inner smooth area. The heat transfer of the leading and trailing surfaces shows an increasing trend with the ascend of the rotation number. At the same time, due to the Coriolis force, the heat transfer of the trailing surface is stronger than that of the leading surface. Results show that the total heat transfer of the four experimental models intensify with the increase of the total cross-sectional area of the pin-fins. In particular, the total cross-sectional area of pin fin is increased by 3 times, and the heat transfer of channel can be strengthened by 46–77%. The effective method to increase the total cross-sectional areas of the pin fins is to increase their diameters.

Heat Transfer and Pressure Loss Correlations for Leading Edge, Jet Impingement Using Racetrack-Shaped Jets With Filleted Edges

Abstract This paper presents an experimental investigation of heat transfer and pressure loss for leading-edge jet impingement using racetrack-shaped jets. The majority of literature for gas turbine cooling applications considers jet impingement using square or sharp-edged orifices. However, the edge of the jet orifices generally has some degree of filleting (or rounding) along the edges, due to casting the airfoils or material wear due to long-term operation. Engineers need data under realistic engine configurations to improve the utilization of coolant while adequately protecting the airfoil. The current experimental study is a parametric investigation of heat transfer and pressure loss for leading-edge jet impingement, where the effects of jet Reynolds number (Re = 10,000–100,000), jet–to–jet spacing (s/d = 2–8), jet–to–target surface spacing (z/d = 2–4), surface curvature (D/d = 2.67–5.33), and jet fillet–to–jet plate thickness (r/l = 0.16–0.5) are each considered. The edge rounding at both the inlet and outlet of the jet plate yields reduced heat transfer compared to the square-edged jets. However, the fillets significantly improve the discharge coefficients associated with the racetrack-shaped orifices. With the extensive testing completed in this study, design correlations have been developed to predict the surface Nusselt number and discharge coefficients, with 10% and 19% deviation from experimental results, respectively. Engine designers can predict the level of heat transfer and pressure loss for leading-edge jet impingement using these correlations.

Experimental Investigation of Heat Transfer in a Flat-wall Impinging Diesel Spray Flame under Diesel-like Conditions

The heat loss through the walls of Internal Combustion Engines (ICEs) needs to be minimized and improved since it has a detrimental impact on the overall thermal efficiency of the engine. This study focuses on investigating heat transfer in a flat-wall impinging diesel spray flame, simulating diesel conditions, to optimize engine parameters. The effects of factors such as various injection pressures, nozzle hole diameters, impingement distances, and oxygen concentrations are analysed in combined and individually. Experimental techniques, such as high-speed imaging, heat flux sensors, and thermocouples, are used to visualize spray flame, measure heat flux profiles and temperature distributions, respectively. Preliminary results and their implications on heat transfer and heat loss are discussed. Regarding the parametric studies investigating the effect on wall heat loss under conditions similar to those of a small diesel engine, it was observed that the transferred heat on the wall was significant in certain conditions. These results emphasize the effectiveness of manipulating the injection rate profile as a viable step to suppress the heat transfer through the wall.

Experimental investigation of heat transfer and effectiveness of employing water and ethylene glycol mixture based Fe3O4 nanofluid in a shell and helical coil heat exchanger

A shell and helically coil heat exchanger (SCHE) is used to experimentally investigate the performance of 60% water and 40% ethylene glycol mixture based Fe3O4 nanofluid as a coolant. Chemical co-precipitation was used for the synthesis of large quantity of Fe3O4 nanoparticles with an average particle size of 11.42 nm. Correlations for the thermo-physical properties of the nanofluid were developed. The exergy analysis and heat transfer characteristics pertaining to the heat exchanger were also investigated. The frictional exergy destruction is found to contribute together with heat transfer exergy destruction to the total entropy of the SCHE system. The exergy efficiency of the SCHE can be increased by 21% using 1.0 vol% of nanofluid, but the improvement becomes modest as the volume concentration is increased further. The NTU of SCHE is improved only by 2% over that of the base fluid at the flow rate of 6 lit/min because of the limited improvement of the overall heat transfer coefficient. Increasing the concentration of the nanofluid beyond 1.0 vol% shows a little improvement in the effectiveness of the heat exchanger. This study recommends that the volume concentration of Fe3O4/60:40 % W + EG nanofluid should not exceed volume concentration of 1.0% because the added benefit of working with concentrations higher than 1.0 vol% may not outweigh the added cost and complexities of the nanofluid.

Experimental investigation of heat transfer to supercritical pressure R410a in a U-tube

To properly guide the design of vapor generators in transcritical organic Rankine cycles, heat transfer characteristics of supercritical pressure R410a in a horizontal U-tube was experimentally investigated. The pipe diameter d and bend diameter D of the U-tube are 4.35 mm and 43.5 mm, respectively. Measurements were performed at mass flux of 300–1600 kg/(m2s), heat flux of 25–80 kW/m2, inlet temperature of 283–343 K, and pressure of 5–5.88 MPa. In all cases, local enhancement of heat transfer was observed within the bend. The extent of bend-induced enhancement is related to Reynolds numbers and property variations of fluid. Up to 25 % increase in the enhancement was observed in the case of dramatic property changes when compared to case with little property change. The bend-affecting upstream length is within 5.75d, and the bend effect diminishes at 11.5d downstream of the bend and further. Moreover, it was shown that a few existing correlations perform reasonably well in the straight sections, but not in the bending section. A new correlation was developed by introducing an enhancement factor to account for the bend-induced centrifugal effect. The newly proposed equation correlates 85 % of the experimental data with an absolute error less than 10 %.

Experimental investigation of heat transfer for nanofluid–porous magnetohydrodynamic thermally driven flow in a novel I-shaped enclosure

Experimental investigation of heat transfer for nanofluid–porous magnetohydrodynamic thermally driven flow in a novel I-shaped enclosure

Experimental investigation of heat transfer in a cavity filled with (50% CuO-50% Al2O3)/Water with hybrid nanofluid attached to a vertically heated wall partially integrated with PCM

An empirical evaluation of free convective heat transmission was conducted in a rectangular enclosure containing a hybrid nanofluid of (50% CuO-50% Al2O3)/water linked to a PCM-containing wall. The enclosure's left and right surfaces were kept at constant warm and cold temperatures, whereas the remaining surfaces were assumed to be isolated. The left side was filled partially with PCM. Several variables were examined, such as the hot-side temperature differential (∆T =10, 15,20 ◦C) and the hybrid nanofluid concentration (Φ=0.03,0.05,0.07)%. The findings show that the rate of heat transmission through natural convection rises as the concentration of nanomaterials rises. Due to its great absorbability and heat storage capacity, PCM was also shown to have the potential to lower the hot side temperature by up to 15.5%. The Nusselt number rises over time as the left cavity is filled partially with PCM. When added hybrid nanofluid is, PCM’s heat-storage efficiency and, by extension, its ability to cool the hot side is greatly improved.

Experimental investigation of rectangular mini channel array as an effective tool for energy efficient cooling of electronic gadgets

ABSTRACT In this paper, an experimental investigation of heat transfer and fluid flow analysis of rectangular mini channel array under steady flow condition is carried out. The channel dimensions are selected in such a way that, flow within the channel is maintained hydro dynamically developed and thermally developing. This type of flow gives enhancement in heat transfer with considerably reduced frictional pressure loss. This study indicates that with a mini channel array maximum heat flux dissipation is extended up to 1.5 MW/m2 at a pressure drop of 8.671kPa. In addition to this, enhancement of heat transfer per unit pumping power (Q/Pp) is also increased by three fold for average heat flux dissipation of 1.25 MW/m2 as against 0.6 MW/m2 at Re = 191. Total minimum thermal resistance reported in this experimentation is 0.128K/W, which is less than the maximum estimated thermal resistance of 0.132K/W for processor having predicted thermal design power (TDP) value of 303. Increase of average heat flux dissipation and reduction in thermal resistance at higher Re (Re > 400) result in substantially reduction in junction temperature Tj(<338K). This results into increase in reliability and sustainability of high heat flux generating electronic gadgets with increase in speed of performing arithmetic computational capabilities.

Experimental investigation of heat transfer and pressure losses across staggered Body Centered cubic arrays fabricated by Laser Powder Bed Fusion

Truss type lattice structures are regarded as a promising approach to design compact heat exchangers that are characterized also by good mechanical performances. The design of these structures can be tailored to satisfy multifunctional requirements and manufactured by exploiting the capabilities of additive manufacturing. This study reports on the experimental characterization of the aerothermal behaviour of new Body-Centred Cubic (BCC) lattice stagger arrays in a rectangular channel, which differ by the truss diameter. The heat exchangers are made from AlSi10Mg by means of Laser Powder Bed Fusion, and the geometric conformity and surface roughness of the internal features were characterized through optical and non-destructive methods to precisely define the boundary conditions at which the aero-thermal performances are provided. By so doing, some marginal defects caused by inherent limitations of the technology were observed, and relevant information in the perspective of designing new structures on a smaller characteristic scale were collected. The samples were then tested on a dedicated test rig by applying uniform heat flux boundary conditions up to a Reynolds number of 20′000. The experimental rig and methodology were firstly tested on a smooth channel and the results were found to be consistent with the literature. The BCC structure with the largest diameter performs the best in terms of heat transfer in view of the higher wetted surface. On the other hand, as expected, the friction factor shows the opposite trend. However, the thermal performance factor does not differ much among the different configurations, with slightly better performances observed for the larger diameter truss. Finally, correlations were found to predict both aero and thermal performances of the considered structure by introducing a modified formulation for the friction factor, Nusselt and Reynolds numbers.

Prediction of effective heat transfer coefficients for vapour condensation inside horizontal tubes in stratified phase flow

In modern condensers of air conditioning systems, heat pumps, evaporators of seawater desalination systems, and heaters of power plants, the process of vapour condensation is carried out mainly inside the horizontal tubes and channels. Heat transfer processes occurring in condensers have a significant effect on the overall energy efficiency of the mentioned systems. In this paper, the experimental investigation of heat transfer during condensation of freons R22, R406a, and R407c in the plain smooth tube with d = 17 mm were carried out with the following parameters: ts = 35–40°C, G = 10–100 kg/(m2s), x = 0.8–0.1, q = 5–50 kW/m2, ΔT = 4–14 K. The unique measurements of circumferential heat fluxes and heat transfer coefficients were carried out with the thick wall method during different condensation modes. It can be inferred that with the increase of the heat flux, at the top part of the tube the thickness of the condensate film increases, which leads to the decrease in heat transfer. At the bottom of the tube, the increase in the heat flux enhances heat transfer coefficient, that is characteristic of the turbulent liquid flow in the tube. The obtained results allowed improving the prediction of effective heat transfer coefficients for vapour condensation, which takes into account the influence of condensate flow in the lower part of the tube on the heat transfer. This method generalises with sufficient accuracy (error ± 30%) the experimental data on condensation of freons R22, R134a, R123, R125, R32, R410a, propane, isobutene, propylene, dimethyl ether, carbon dioxide, and methane under stratified flow conditions. Using this method for designing heat exchangers, which utilise such types of fluids, will increase the efficiency of thermal energy systems.

Experimental investigation of heat transfer in bio-based building block with microencapsulated PCM

The paper presents the investigation of heat transfer in hemp shives and magnesia binder composite with microencapsulated phase change material (PCM) to increase the building material’s thermal mass. The investigated sample was prepared as a three-layer block of dimensions 600×400×480 mm. The external layers of the block had a density of approximately 400-450 kg/m3 and a thickness of 50-60 mm, while the inner layer had a density of approximately 200-250 kg/m3 and a thickness of 290 mm. The study was performed with a block placed between two specially designed climatic chambers. Conditions in chambers were fully stabilized. Additionally, 50 mm wide channels with fans in both chambers were added close to the sample surfaces to force uniform airflow along both sample surfaces. For parameters control in the chambers, dedicated software was developed. Inside the sample nine temperature sensors were placed. During measurements, the temperature of 30°C and relative humidity of 40% was maintained in one chamber, while in the second one, after stabilization in temperature of 20°C and relative humidity of 50%, the temperature was decreased to 15°C without humidity control. During measurement insulating properties of PCM in dynamic conditions were analyzed.

Numerical and experimental investigations of heat transfer inside a rectangular channel with a new tilt angle of baffles for solar air heater

This work represents an experimental and numerical study of heat transfer by forced convection inside a channel containing the baffles of a solar collector. The study chose the shape of the baffles as an important factor to improve heat exchange, which has a rectangular shape and is transversal with air flowing at an angle of inclination ? = 90 degrees. The study was conducted at different mass flow rates and different times of the day, to find out the effect of these conditions on the convective heat transfer from the absorber plate to the air through the channel of the collector. The operating conditions taken from the experiment were entered as boundary conditions in CFD, for a comparative study between the heat transfer coefficient by convection of the measurement data, and the simulation data. It was found that the results of it in the numerical and experimental methods gave a good approach, also it can be concluded that this coefficient was affected by different parameters such as the mass flow rate, absorber temperature, and shape of the baffles. Through the results, it was confirmed that when the Reynolds number increases, it means an increase in velocity, which means that the air passing through the duct becomes cooler, therefore there is a difference in temperature between the passing air and the absorber plate, and this leads to an increase in heat transfer between the air and the absorber plate.

Experimental investigation of heat transfer and flow analysis of artificially roughened solar air heater by using double arc reverse shaped roughness with gap on the absorber plate

Artificial roughness in the form of ribs is a convenient method for the enhancement of heat transfer coefficient in a solar air heater. In the present paper, an indoor experimental examination of Nusselt number (Nu) and friction factor (f) characteristics of a Solar air heater using novel discrete double arc reverse form roughness bottom side of the absorber plate was carried out. The range for criterion, relative roughness pitch (p/e) of 6.67, relative roughness height (e/Dh) of 0.027, angle of arc (α) of 30°,45°, 60°, 75°, Reynolds number (Re) of 3000 to 14000, aspect ratio (W/H) of 8 were applied in between the laboratory test. It has been found that the performance of roughened solar air heater duct is better than the performance of a smooth duct for the range of roughness parameters investigated. Enhancement of heat transfer coefficient (h) was noticed 266% more than a smooth channel. Increments of Nusselt number (Nu) and friction factor (f) were found at 268% and 221% more than smooth channel respectively. Maximum thermal efficiency was found 137% more than a smooth channel. Thermo-hydraulic concepts have been used to identify the ideal design and operating conditions. Depending on the insolation, it has been discovered that systems running within a certain Reynolds number range operate better thermo-hydraulically. The range of Thermo-hydraulic performance (THP) improved along with the increment of the Reynolds number. At a relative roughness pitch (p/e) of 6.67, a relative roughness height of 0.027, along the angle of arc (α) of 60°, the highest improvement of Thermo-hydraulic performance (THP) was noticed 2.22.

Parametric Study and Correlation of Racetrack Shaped Jets for Leading Edge Impingement

Abstract This paper provides a detailed experimental investigation of heat transfer and pressure loss for leading edge jet impingement using square-edged, racetrack jets. Experiments were carried out over various parameters: jet-to-jet spacings (s/d) of 2, 4, 8; jet-to-target surface spacings (z/d) of 2 and 4; jet plate thicknesses (l/d) of 1.33, 2.6, and 4; and Reynolds numbers from 10,000 to 100,000. The results show that pressure loss and heat transfer decrease with increasing s/d and z/d. There is an interesting observation for jet plate thickness at l/d = 2.6—a local minima for both heat transfer and pressure loss is measured with the racetrack shaped jets. Using the exhaustive experimental data, design correlations are developed to estimate the surface Nusselt number and discharge coefficients within the domain of geometric and flow parameters. The novel correlations account for the parametric effects and can accurately predict the Nusselt number and discharge coefficient with deviations of 19.8% and 15.9%, respectively. These provide a capability for the engine designers to predict the heat transfer and pressure loss for leading edge jet impingement.