Experimental Study Of Heat Transfer Research Articles

Theoretical and experimental studies of heat transfer in a double-pipe heat exchanger equipped with twisted tape and nanofluid

This study represents the performance of Fe2O3/water nanofluid (20 nm) in a double-pipe heat exchanger equipped with twisted-tape inserts for enhancing heat transfer. Fe2O3/water nanofluid is used because it has a higher thermal conductivity. Considered parameters include mass flow rate, twist ratio of tape, temperature and the volumetric portion of nanoparticles to water. The nanoparticles volume concentrations are 0.08% and 0.1% (v/v), and different twisted tapes with twist ratios of 2.5 ≤ y/w ≤ 5.2 are used. The Reynolds number variation is within the turbulent flow regime of 5000 < Re < 28,500. The obtained results show that adding nanoparticles as well as twisted-tape inserts increases heat transfer and Nusselt number. However, the effects of nanoparticles are more pronounced in high Reynolds number flows. Combining the positive effects of nanofluid and twisted tape, Nusselt number is significantly improved up to 103.45% in the test case. Moreover, there is no major change in friction factor. A multilayer perceptron (MLP) artificial neural network (ANN) with Levenberg–Marquardt (LM) learning algorithm and tangent sigmoid nonlinear transfer function is implemented for the aim of modeling the Nusselt number.

Experimental study of heat transfer at the transition regime between the natural convection and nucleate boiling: Influence of the heated wall tilt angle on the onset of nucleate boiling (ONB) and natural convection (ONC)

The works carried out in this study aimed to get a better understanding of the heat transfer in natural convection and nucleate boiling as well as the transition between these two regimes. An experimental test set up was built to generate the liquid boiling on the wall using a boiling-meter. This sensor was developed in order to investigate the heat transfer and its dependence with time in well controlled conditions. Temperature, pressure and heat flux required for the measurements were implemented. Experiments were focused on determining the characteristic curves of heat transfer in the natural convection regime and the nucleate boiling regime. The influence of the orientation of the wall relative to the gravity on the heat transfer is investigated. Characteristic heat transfer curves were obtained in several operating conditions and wall orientation with regard to gravity.At the transition regime between natural convection and nucleate boiling regimes, it was highlighted a competition between these two regimes. It was found, that the transferred heat flux differs depending on the orientation of the wall and the degree of liquid superheat in the nucleate boiling regime. In the natural convection regime, the measured heat fluxes exhibited a slight variation with the tilt angle of the wall. While in the nucleate boiling regime, strong variations were observed on the onset of nucleate boiling as well as for the heat flux. At very low heat flux, the heat transfer increases with the tilt angle while it decreases for moderate heat flux. The onset of nucleate boiling (ONB) and the onset of dominant natural convection (ONC) were found to decrease as a function of the tilt angle of the wall with different variation laws. The wall superheats values for the ONB and ONC are getting closer when the tilt angle tends to 180°.

Experimental study of single-phase flow and heat transfer in rectangular channels under uniform and non-uniform heating

Characteristics of single-phase convective heat transfer in a rectangular channel are reported for heated and non-heated conditions. Results indicated that the friction factors were basically the same in the laminar region. However, in the transition and quasi-turbulent region, the heated friction factor was slightly lower than the non-heated values, and the value of non-uniform heating was slightly higher than that of uniform heating. The transition started critical Reynolds number increased from 2700 without heating to about 3000 with heating only due to the viscosity decreased with increasing temperatures. Due to the transition started and ended Reynolds number increased simultaneously under heating, the width of flow transition was almost the same as that without heating. The transverse power distribution influenced the average Nusselt number of the channel, although measured differences were small for uniform and non-uniform heating conditions. For the heat transfer data under uniform heating conditions obtained in this study, 15 correlations were evaluated. It is found that the existing correlations underestimated the experimental data for Re < 3000 and overestimated the data for 3000 < Re < 6000. A large database of heat transfer in rectangular channels including the experimental data obtained from this study and other researchers' work was established, and new heat transfer correlations for laminar (Re < 3000) and transition (3000 < Re < 10000) flows were developed. The Dittus-Boelter correlation was confirmed to provide a good prediction for the heat transfer for Re > 10000, with almost all data being within ±20% errors.

Experimental Study of Flow Boiling Heat Transfer of R30 in Microchannels

Two-phase cooling technique including flow boiling heat transfer in microchannel is being widely applied in the mechanical and electronic devices, whereas most of the existing related studies still experiment with common coolants with low boiling points that are not suitable for extreme high-temperature heat dissipation. Thus the flow boiling heat transfer characteristics of R30 have been analyzed through an experiment in parallel microchannels with diameters of 0.5 and 1.0 mm. The experiment was performed under the following conditions: heat flux, ; mass flux, ; outlet vapor quality, 0–0.52. Results showed that the heat transfer coefficient is determined by heat flux and vapor quality in low-vapor-quality region dominated by nucleate boiling, and affected by vapor quality and mass flux in high-vapor-quality region dominated by convective boiling. Intermittent dryout occurs in the nucleate boiling region and is affected by mass flux resulting from the suppression of nucleate boiling. Higher mass flux can relieve the intermittent dryout and postpone the onset of the dryout incipience to higher vapor quality. Comparison with previous correlations showed their unsatisfactory accuracy on present data and implied relatively well heat transfer performance of R30.

Experimental and numerical heat transfer study of turbulent tube flow through discrete V-winglets

The article presents an experimental and numerical investigation on heat transfer augmentation in a tubular heat exchanger inserted with discrete-V-winglet (DW) tape. Air entered the DW-inserted tube having a uniform wall heat flux in a turbulent flow regime, Reynolds number from 4200 to 25,800. Two arrangements of DWs: V-tip pointing upstream (V-up) and downstream (V-down) were introduced with three relative winglet heights (RB=b/D = 0.1, 0.15 and 0.2) and four relative winglet-pitches (RP=P/D = 0.5, 1.0, 1.5 and 2.0), all at a single angle of attack, α = 30°. Effects of those parameters on the heat transfer/Nusselt number (Nu) and friction factor (f) were examined. Also, a novel thermal-performance enhancement factor (TEF) was put forward. The experimental result has shown that at a given RB, the smallest pitch length (RP=0.5) yields the highest f and Nu. The DW with RB=0.2 and RP=0.5 has the maximum Nu and f of about 3.8 times and 18.8 times, respectively whereas the one with RB=0.15 and RP=1.0 gives the highest TEF of about 1.99 and 2.02 for the V-up and V-down, respectively. Moreover, the DW provides higher TEF than the typical V-winglet as expected. To explore the mechanism of heat transfer, a numerical model of the inserted-tube flow was carried out and the numerical result was validated and found in favorable agreement with available measurement. Flow structures and heat transfer patterns from the simulation such as temperature contours and streamlines including the Nusselt number contours were also proposed.

Design of in vitro model to study oral aroma release: Experimental study and numeric simulation of heat transfer in a foamed dairy matrix

Since aeration can increase aroma perception, it may be used to compensate the perceived aroma loss of low-fat dairy products. To be able to study the effect of aeration on aroma release systematically in the future, a temperature study was conducted on an in vitro model built to study aroma release at temperatures relevant for oral processing. The studied dairy foam contained 4% (w/w) milk protein and 0.75% (w/w) alginate and had a gas volume fraction of 71% after whipping. The temperature development during incubation of the foam at 40 °C was calculated using a numeric simulation. Thermal equilibrium was reached after 50 min. Sampling for dynamic aroma release was found to be ideal at 2, 7 and 15 min, because mean foam temperatures were 25, 30 and 35 °C. Experimental temperature analysis of the foam core was in good agreement with the simulation (R2 = 0.95).

Suppression of Flaming Combustion and Thermal Decomposition of Condensed Matter at Different Heights of the Beginning of Water Array Motion

Results of an experimental study of heat and mass transfer and phase transitions during suppression of flaming combustion and thermal decomposition of condensed matter (CM) by an example of typical combustible materials in the forest (mixtures of leaves, conifer needles, and twigs) by means of the action of a non-atomized water array at different stages of its transformations are reported. The liquid volume and the height of its discharging with respect to the CM surface are varied with allowance for the known stages of water array transformations to illustrate possible differences in the cross-sectional areas of these arrays, which determine the water–CM contact area. The characteristics of CM burning termination are obtained as functions of the height of the beginning of water array motion.

An experimental study of mechanical properties and heat transfer of acrylic composites with structural and surface modified Al2O3 particles

Thermal properties of acrylate-based composite combined with alumina-based particles were investigated. The composites were made of poly (methyl methacrylate) (PMMA) modified with dimethyl itaconate (DMI) as a matrix. Neat alumina particles (Al2O3) and alumina doped with iron oxide (Al2O3-Fe) modified with 3-aminopropyl-trimethoxylane (AM) and flax oil fatty acid methyl esters (biodiesel - BD) were used as reinforcement. Thermal conductivity measurements showed that composite with alumina particles with the highest alpha phase content had the highest thermal conductivity values. Mechanical properties (tensile strength, modulus of elasticity and elongation at break) were improved by the addition of modified alumina particles to PMMA/DMI matrix. The thermal properties are the most improved for composite with 3 wt.% Al2O3-Fe-AM particles that had the lowest temperature in a crack zone was noticed (10.28 %) compared to PMMA added due to the better stiffness and highest thermal conductivity.

HEAT TRANSFER OF FLAT-OVAL TUBE BUNDLES IN CROSS FLOW

The goal of this work was to create generalized relationships for calculating the convective heat transfer coefficients of staggered bundles of flat oval tubes based on the obtained experimental data for recuperative heat exchangers of the “gas-gas” type in a wide range of changes in the geometric characteristics of tubes, bundles and flow regime parameters. An analysis of the known literature data by the authors shows that no systematic studies of the influence of geometric and step characteristics of tubes on heat transfer and aerodynamic drag of flat-oval tube bundles during transverse air flow in a wide range of Reynolds numbers were carried out. The results of numerical modeling are not experimentally confirmed. The published works contain fragmentary data, which are usually of carried a private character. Presented experimental studies of convective heat transfer of staggered bundles of flat oval tubes with transverse air flow around them were carried out in the range of Reynolds numbers 2000 &lt; Red1 &lt;30000 with a variation in the relative elongation of the profile of the flat oval tube d2 / d1 from 2 to 5, and the ratio of the transverse tube pitch to the longitudinal S1 / S2 ranged from 0,4 to 1,45. The influence of geometric and operational parameters on the heat transfer of bundles is established. The heat transfer rate of staggered bundles of flat-oval tubes when varying the elongation of the tubes profile d2 / d1 in the range from 2 to 5 changes by (10-25) %, and when varying the step characteristics of the tubes at d2 / d1 = const by (8-12) %. Evaluation of the accuracy of the generalizing formulas, which was carried out on the basis of an array of experimental data for the 49 studied bundles, indicates that the difference between the experimental and calculated values of the Nusselt numbers does not exceed ± 10%. Design relationships are proposed for determining the heat transfer coefficients of staggered bundles of flat-oval tubes, taking into account the influence of the operating parameters on the heat transfer, the geometric characteristics of flat-oval tubes and bundles and can be used in calculating the heat transfer of air heaters of boiler units, gas turbine plant regenerators.

Experiments on mixed convection in a vented differentially side-heated cavity filled with a coarse porous medium

This paper reports on an experimental study of mixed convection flow and heat transfer in a vented, differentially side-heated cubical cavity filled with a porous medium consisting of relatively large solid low-conductivity spheres. Rayleigh numbers and Reynolds numbers are varied over the ranges 6 × 106 < Ra < 7 × 107 and 240 < Re < 4250, respectively, for a fixed Prandtl number of Pr = 6.75, thus covering more than three decades in Richardson numbers Ri = Ra/(Re2 Pr). Heat transfer measurements were combined with measurements of the velocity field (using particle image velocimetry) and the temperature field (using liquid crystal thermography) to better understand the dependence of the Nusselt number, Nu, on the Richardson number. We observed three different flow and heat transfer regimes depending on the Richardson number. For Ri < 10, the flow structure and the Nusselt number scaling are similar to those for the pure forced convection, i.e., the Nusselt number scales as Nu ~ Re0.61 independent of Rayleigh number. For Ri > 40, natural convection dominates the flow in the vicinity of the heating wall. The Nusselt number becomes less sensitive to the Reynolds number and is mainly determined by the Rayleigh number. In the intermediate regime for 10 < Ri < 40, the upward directed natural convection flow at the heating wall competes with the downward directed forced flow leading to a minimum effective Nusselt number. A Nusselt number correlation is derived that is valid in the range 0.1 < Ri < 100 covering all three regimes.

Experimental study of free-surface jet impingement heat transfer with molten salt

An experimental study of free-surface jet impingement was carried out with compound molten salt to understand its characteristics of heat transfer. Based on pre-experimental verification on the reliability of experimental method by water as working fluid, a compound molten salt was chosen as a new working fluid to conduct experiment of free-surface jet impingement. The heat transfer of molten salt at stagnation point was investigated in detail, and the Nusselt number of molten salt increased with increasing of the Reynolds number and the Prandtl number, and was nearly constant with increasing of nozzle-to-plate spacing at present ranges. The data was correlated by an empirical formula, which was compared with those classical correlations from references. Next, the variation of the Nusselt number of molten salt was also verified along heated surface. It was shown that the local Nusselt number decreased monotonously with increasing of lateral distances from stagnation point, and increased with increasing of the Reynolds number. The profiles of the Nusselt number were nearly unchanged for different nozzle-to-plate spacing, which can be well correlated by a normalized formula. This work focused on the heat transfer characteristics of the molten salt in free-surface jet impingement, which may be recognized as a reference for promoting the application of molten salt in heat transfer process, especially in high temperature situations.

Experimental and numerical study of heat and mass transfer during contact heating of sliced potatoes

Studies concerning the contact heating of food products remain relatively rare in the literature despite the importance of this mode of heat transfer in many operations (grilling, pan-frying).To deal with this, kinetics of product water loss and temperature rise were recorded during contact heating of potato slices in order to examine the influence of the heating power and of the presence or not of an oil layer below the heated product. From the experimental data acquired, a 2D mathematical model based on a moving boiling-front approach was developed and validated allowing the determination of contact heat transfer coefficient values of 512.2 ± 12.4 W m−2.K−1 and 197.5 ± 5.8 W m−2.K−1 for experiments with and without oil respectively. The analysis of the simulation results showed that the overall heating of the product is limited by: (i) the evaporation of liquid water at the location of a boiling front propagating within the heated product and (ii) by the development of a dried region (crust) in the lower part of the product acting as a thermal insulating layer. It should also be noted that the determination of the contact heat transfer coefficient can become an incidental problem (especially for experiments with oil) since the thermal contact resistance is often much lower than the thermal resistance associated with conduction in the dried region of the product.

An experimental study of hydrodynamics and heat transfer at fluid filtration through a porous medium

An experimental study of hydrodynamics and heat transfer under conditions of filtering of water and aqueous solution of glycerol through a porous insert in the channel was carried out. The values of the coefficient of hydraulic resistance and heat transfer for a chaotic backfill of balls with a diameter of 3.2 mm for filtering water and 47% glycerol solution have been determined. Three flow regimes were investigated: inertial, transient and turbulent. It has been found that, depending on the filtering mode through the porous insert, there are various laws of heat transfer. Comparison of the obtained results on heat transfer in ball fillings with the available literature data has been carried out. The obtained experimental data are in good agreement with the data of numerical calculations. For the thermal stabilization section at high values of the Reynolds number they are in good agreement with the data obtained by filtering air through a porous medium.

Experimental study of the control agent flow parameters in the channel, simulating the ramjet air flow duct

This article presents the results of the experimental study of heat transfer when the supersonic air flow interact with the surface of the ramjet model air flow duct in the range of Mach numbers M = 5 – 7. Experimentally obtained temperature distribution along air flow duct of a ramjet shown. The temperature along the axisymmetric model of the air flow duct was measured using the designed hot probe. Distribution of the Mach number along the symmetry axis of the model air flow duct is obtained numerically. A comparison of the numerical and experimentally obtained values of the Mach number showed their satisfactory agreement

Experimental research of supersonic flow heat transfer in the wake of a backward-facing step

The results of an experimental study of heat transfer for a supersonic flow behind a backward-facing step are presented. Input flow Mach number was 2.2, Reynolds number was over 20 million when calculated with the use of the dynamic boundary layer length from the critical section to the exit section of the nozzle. The height of the step ranged from 8 to 14 mm. The thickness of dynamic boundary layer for plane model flow was about 6 mm in the region of nozzle exit section. The heat transfer coefficient and adiabatic wall temperature for the wake flow behind the step have been analysed in comparison with the plane model flow. The experiment was made with the use of chromel-kopel thermocouples with thermal compensation of cold junction, stagnation and static pressure meters, NI LabView powered programs of experimental automation. The graphs of the wall temperature, stagnation flow temperature, adiabatic wall temperature, the stagnation and static pressure, the distribution of the temperature recovery coefficient and the relative Stanton number along the model length are presented.

Heat transfer at evaporation/boiling in the thin horizontal liquid layer on microstructured surfaces under low pressures

The results of the experimental study of heat transfer during boiling of n-dodecane on a capillary-porous surface are presented. The surface is coated using a 3D laser printer. The dependence of the heat transfer coefficient on the heat flux density under different pressures and liquid layer heights is studied. The obtained experimental data were compared with the values obtained during evaporation/boiling on a plain surface under the same conditions.

Experimental study of heat transfer under flow boiling ozone-safe refrigerants in microchannels

This paper devoted to experimental measurement of heat transfer and critical heat fluxes into horizontal microchannels under flow boiling of refrigerant 141b. The copper experimental test section contains two microchannels with a 2x0.36 mm cross-section. Average heat fluxes versus wall superheat in the range of mass fluxes from 600 to 2100 kg/m2s and heat fluxes up to 140 W/cm2 are presented. A slight deterioration in heat transfer relative to pool boiling with heat fluxes close to critical was observed.

Experimental study of heat transfer in a heat pipe

Heat pipes are a promising cooling system due to the fact of high efficiency and non-availability of moving parts such as pumps. The experimental investigation of heat transfer in a flat heat pipe has been carried out. The temperature field on the heat pipe has been evaluated using an infrared camera and K-type thermocouples. The IR-camera data and K-type thermocouple data have been compared. The linear dependence of the power supplied to the evaporator on the temperature difference between the evaporator and the condenser is shown.

Heat coupling effect on photothermal detection with a moving Gaussian excitation beam.

A strong photothermal response is beneficial to the measurement of optical and thermal properties of optical materials using the laser-induced thermal mirror method. A highly sensitive asymmetrical thermal mirror method was recently proposed by employing a moving Gaussian excitation beam [Appl. Phys. Lett.114, 131902 (2019)APPLAB0003-695110.1063/1.5080163]. However, the heat transfer across the interface between the thermodynamic system and the surroundings is ignored, which will lead to an error in the absolute measurement of the material properties. To address the problem, we present a theoretical and experimental study of heat transfer within the heated sample and out to the air coupling fluid in the photothermal detection with a Gaussian excitation beam moving at a constant velocity. We analyze the dynamic temperature fields inside the sample and in the surrounding air, and the phase shifts induced by the thermoelastic displacement of the sample (thermal mirror) and the refractive index gradient of air (thermal lens), as well as the diffracted intensity profiles of the probe beam in the detection plane. The experiments are implemented under normal pressure and vacuum, respectively, for a fused silica glass-air heat coupling system to verify the theoretical model. The experimental results show that the thermal lens, due to the heat coupling effect, introduces a signal deviation approximately 4.2% of the total photothermal signal, which is close to the theoretical result of 5%.

Experimental study of hydrodynamics and heat transfer in two-phase natural circulation loop with reference passive cooling systems of nuclear power plants

In spite of long year practical use of natural circulation systems and not less long period of study working characteristics of these systems the problem of reliable predicting calculations of natural circulation loops (NCL) with boiling coolant at low reduced pressures is not likely can be considered as being solved. At the same time the interest to solution of this problem arises today, because low pressure natural circulation loops are considered as main type of post accident passive cooling systems for nuclear power plants. The main difficulties for calculations are concerned with determining friction pressure losses, which are strongly depended on two-phase flow pattern. At low reduced pressures large difference between liquid and vapour (gas) specific volumes predetermines the strong change of flow pattern even at small change of mass flow quality. An attempts to calculate local two-phase flow parameters (void fraction, circulation velocity) in the wide range of mass flow qualities without considering the change of flow patterns often lead to large deviations of calculated values from the results of experimental measurements. Another specific feature of two-phase low pressure NCL is hydrodynamic flow instability with circulation velocity pulsations of high amplitude and the occurrence of reverse flows. This also presents problems in developing calculation method. In present paper a method for calculating a low-pressure NCL has been developed, in which local two-phase flow parameters (void fraction, the phases velocities, pressure) are calculated using a modified homogeneous model with taking into account the distribution factor and the phases slip and a model of an annular-dispersed flow with considering droplets entrainment and deposition. A modified homogeneous model was applied for description quasihomogeneous flow regimes. At high values of void fraction annular-dispersed flow model was used. Recommendations for change from one model to another in practical calculations have been formulated and verified. The proposed calculation method has been verified by the comparison the calculated thermo-hydraulic characteristics of laboratory natural circulation loop with the experimental data. The experiments have been carried out for boiling in the loop of three different liquids – water, ethanol and perfluorohexane (FC-72) at atmospheric pressure. The comparison of calculated and experimental results showed their good agreement.