Experimental Investigation of Transient Forced Convection of Liquid Methane in a Channel at High Heat Flux Conditions

Transient heat transfer of liquid methane under forced convection in a 1.8 mm × 1.8 mm asymmetrically heated square channel was investigated. This study is aimed at understanding the heat transfer behavior of cryogenic propellant in cooling channels of a regeneratively cooled rocket engine at the start-up condition. To simulate high heat load conditions representative of regeneratively cooled rocket engines, a high heat flux test facility with cryogenic liquid handing capabilities was developed at the Center for Space Exploration Technology Research. The time history of inlet and outlet fluid temperatures and test section channel wall temperatures were measured at high heat flux conditions (from 1.19 to 3.80 MW/m²) and a Reynolds number (Re) range of 1.88 × 10⁵ to 3.45 × 10⁵. The measured wall temperature data point toward possible film boiling within the test section during certain tests, particularly with higher heat fluxes and lower Reynolds number conditions that resulted in higher wall temperatures. The transient average Nusselt numbers (Nu_L) of the channel obtained from the experimental measurements are lower than those calculated from the Sieder–Tate correlation (Nu_O); however, the ratio (Nu_L/Nu_O) increases with the increase in Reynolds number. The ratio is around 0.25 at the lower end of Re and then increases to 0.7 at the maximum Re studied in the present investigation.     

Heat transfer and friction factor correlation development for double-pass solar air heater having V-shaped ribs as roughness elements

In this present study, an experimental investigation has been carried out for a roughened double-pass solar air heater. The respective variation in other parameters, such as relative roughness pitch (p/e), relative roughness height (e/D_h), and angle of attack (α), were 5–20, 0.022–0.044, and 30°–75°, respectively. The effect of roughness and operating parameters on Nusselt number (Nu) and friction factor (f) has been determined, and the results are compared with those of a smooth absorber plate; considerable enhancement in both heat transfer and friction factor is noticed. Utilizing these experimental data, correlations for Nusselt number and friction factor correlations were also developed.     

Experimental analysis of conical baffles with rifts on heat transfer and pressure drop

In the present study, convective heat transfer to the air from a heating tube attached to conical baffles with rift was experimentally examined. The air entering the test section first contacts the large surface of the conical baffle. Therefore, the conical baffle both directs the air toward the heating surface and increases the heat transfer surface area. In the experiments, baffles with inclination angles of 45°, 60°, and 80° were used. The baffles were placed on the heating tube at the pitch of 15 mm. The temperature of the heating fluid (water) was kept fixed at 65°C. In addition to the riftless baffles, the experiments were carried out by using baffles with a rift spacing of 1.5 and 3.5 mm so that the boundary layer separation mechanism could be accelerated. Experimental results for eight different velocities of airflow (2–20 m/s) were presented. For the inclination angle of 60°, the increase in the heat transfer of the baffle with rift was 13% at a rift spacing of 1.5 mm and 4% at a rift spacing of 3.5 mm according to the riftless baffle. In addition, for the inclination angle of 60°, the pressure drop values of the riftless and the rift spacing of 1.5 and 3.5 mm were almost the same.     

Stagnation Region Heat Transfer for Circular Jets Impinging on a Flat Plate

The heat transfer characteristics in a stagnation region were investigated experimentally for five circular free jets impinging into a heated flat plate. The local temperature distributions are estimated from the thermal images obtained from an infrared camera. To get a precise heat transfer data over the plate, fully developed straight pipe jets were used in this study. Mean jet Reynolds number varied from 1,000 to 45,000, jet-to-plate vertical non-dimensional distance H/D varied from 2 to 6, and the spacing distance jet-to-jet S/D varied from 2 to 8. A geometrical arrangement of one jet surrounded by four jets an in-line array was tested. The results show that the stagnation point Nusselt number is correlated to a jet Reynolds number as Nu_st∝Re^0.61. The average Nusselt number is higher at a separation distance of 2D for three cases of spacing distances, S/D = 2, 4, and 6.     

Heat transfer and pressure drop measurements in channels roughened by variously shaped ribs on one wall

Heat transfer and pressure drop measurements were conducted to study the thermal-hydraulics in a square, round-edged channel roughened by ribs (e/D_h = 0.0638, p/e = 10) on one wall at Reynolds numbers ranging from 5.0 × 10⁴ to 2.5 × 10⁵. Three variously shaped ribs were investigated: Transverse ribs with square cross sections, transverse ribs, and upstream directed 60° V-shaped ribs with round-edged rib front and rear surfaces. Friction factors, Nusselt number ratios, roughness functions, and the thermal performance were presented. The highest heat transfer and best thermal performance is reached by the upstream directed V-shaped ribs.     

An experimental study on convective heat transfer performance of steam and air flow in V-shaped rib roughened channels

An experimental study on heat transfer characteristics of steam and air flows in a V-shaped ribbed channels was conducted. The effects of Reynolds numbers and rib angles on heat transfer of steam and air were obtained. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were 13.9%, 20.6%, 27.1%, and 27.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively. The correlations for Nusselt number in terms of Reynolds number and rib angle for steam and air in V-shaped ribbed channels were developed.     

Experimental investigation of heat transfer from different pin fin in a rectangular channel

In this study, the effect of both hexagonal pin fins (HPFs) and cylindrical pin fins (CPFs) into the rectangular channel on heat transfer augmentation, Nusselt number and friction factor were experimentally investigated. In planning of the experiments, different Reynolds number, pin fin array, pin fin geometry and the ratio of the distance between pin fin spacing (s) to the pin fin hydraulic diameter (s/D_h) were chosen as the design parameters. Air was used as the fluid. The Reynolds number, based on the channel hydraulic diameter of the rectangular channel, was varied from 3188 to 19531. In the experiments, the heating plate was made of stainless steel foil. The foil was electrically heated by means of a high current DC power supply to provide a constantly heated flux surface. The heat transfer results were obtained using the infrared thermal imaging technique. The heat transfer results of the hexagonal pin fins (HPFs) and cylindrical pin fins (CPFs) are compared with those of a smooth plate. Best heat transfer performance was obtained with the hexagonal pin fins. The maximum thermal performance factor ((?), was obtained as Re = 3188, staggered array, s/D_h = 0, ? = 2.28.     

Theoretical and Experimental Heat and Mass Transfer Analysis for a Falling Film Outside a Grooved Tube

The heat and mass transfer from a grooved tube is investigated experimentally for a falling-film flow. The experiments are conducted on a helical trapezoidal grooved tube at three temperatures of the feeding water: 30°C, 35°C, and 40°C. The Reynolds number (Re) of the air ranges between 1,500 and 3,500. Nusselt number (Nu) is expressed as a function of the Prandtl number (Pr), Re for air (Re_a), Re for water (Re_w); Sherwood number (Sh) is expressed as a function of the Schmidt number (Sc), Re_a, and Re_w, and the correlation coefficients are determined.     

Natural Convection from a Corrugated Heated Surface at the Bottom of Vented Rectangular Enclosure

Natural convection heat transfer characteristics enclosures from a tilted rectangular enclosure heated at the corrugated bottom wall and vented by uniform slots opening at the top wall are experimentally investigated. The experiments were carried out to study the effects of the angle of opening of the corrugated surface, opening ratio, enclosure's tilt angle, and Rayleigh number on the passive cooling of the enclosure. The experiments were carried out at Rayleigh numbers ranging from 2 × 10⁸ to 1.52 × 10⁹ for enclosure tilt angles ranging from 0° to 90° and angle of opening of corrugated surface ranging from 0° to 25°. The top venting arrangement was studied at different opening ratios of 1, 0.75, 0.5, and 0.25. The results gave an optimum angle of opening of the corrugated surface at which Nusselt number is maximum.     

Dynamics and thermodynamics of co-flow non-premixed methane–air flame in a cylindrical micro combustor with heat recirculating wall

Numerical studies have been made on co-flow non premixed methane air flame in a cylindrical micro combustor with heat recirculating wall. An optimum Peclet number Pe_opt based on air flow velocity has been observed at which the flame location from the combustor inlet is minimum and the rate of heat recirculation to fresh charge is maximum. The optimum Peclet number is almost independent of the outer wall Nusselt number Nu_E and the thermal conductivity ratio k_s/k_g. The rate of heat recirculation shows contrasting increasing and decreasing trends with Nu_E, depending upon whether Pe < Pe_opt or Pe >Pe_opt respectively.

A higher limit of Peclet number for flame blow-off and a lower limit of Peclet number for flame extinction have been identified for a given global equivalence ratio and given values of Nu_E, k_s/k_g and t/D (the ratio of wall thickness to tube diameter). The upper Pe limit decreases sharply with Nu_E, while the lower Pe limit is almost uninfluenced by both Nu_E and k_s/k_g. Heat diffusion and chemical reaction in the gas phase are found to be the dominant processes of exergy destruction. The second law efficiency shows a minimum value around 30% at an optimum value of Peclet number where flame stand-off becomes minimum.     

Analysis of ultrasound-assisted convective heating/cooling process: Development and application of a Nusselt equation

In this study, the convective heating/cooling process assisted by US irradiation is analyzed with the aims of developing a new convective heat transfer correlation. Heat transfer experiments were conducted with different copper machined geometries (cube, sphere and cylinder), fluid velocities (0.93–5.00 × 10⁻³ m/s), temperatures (5–60 °C), and US intensities (0–6913 W/m²) using water as heat transfer fluid. The Nusselt (Nu) equation was obtained by assuming an apparent Nu number in the US-assisted process, expressed as the sum of contributions of the forced convection and cavitation-acoustic streaming effects. The Nu equation was validated with two sets of experiments conducted with a mixture of ethylene glycol and water (1:1 V/V) or a CaCl₂ aqueous solution (30 g/L) as immersion media, achieving a satisfactory reproduction of experimental data, with mean relative deviations of 17.6 and 17.8%, respectively. In addition, a conduction model with source term and the proposed correlation were applied to the analysis of US-accelerated heating kinetics of dry-cured ham reported in literature. Results demonstrated that US improves heating of ham slices because of the increased heat transfer coefficients and the direct absorption of US power by the foodstuff.     

Wall heat transfer in static and rotating 180° turn channels by quantitative infrared thermography

The aim of the present study is to develop a new experimental methodology that allows one to perform accurate measurements of the local heat transfer distribution before, in, and after a 180° sharp turn in static and rotating channels. Preliminary measurements of convective heat transfer coefficients are performed by means of infrared thermography applied to the steady state ‘heated-thin-foil’ technique. Some preliminary results in terms of Nusselt number Nu distributions and profile, as well as averaged Nu profiles along the channel axis, are presented. Results prove that infrared thermography is capable of measuring heat flux coefficients and detecting particular phenomena linked to the fluid flow configuration such as location of separation bubbles, influence of the channel aspect ratio as well as the influence of the channel rotation.     

Experimental and Numerical Study of Free Convection on an Isothermal Downward Cone

Abstract

The laminar free-convection heat transfer from an isothermal downward cone in air is investigated experimentally and numerically. The experimental investigation is carried out by Mach-Zehnder interferometery technique and the numerical simulation was done by Fluent. The cone tip angle has been kept constant to 45° and it was suspended from its base throughout the experiment. This article focuses on the effect of Rayleigh number variation on the local and average free-convection heat transfer coefficient over the conical surface. The local and average Nusselt numbers were determined for the Rayleigh number range of 4.9×10⁵ to 1.1×10⁶. Also, the experiment and the numerical simulation were carried out on a vertical isothermal cylinder of circular cross section in order to compare results with other researchers for the verification of our experimental and numerical results. The significant influence of the upper end surface of the cone, both in the experimental and numerical studies, indicated a recirculation region above the upper end surface which affects the local convection heat transfer at the slant trailing edge and causes it to increase. Also, a correlation for the calculation of the local Nusselt number over the cone is proposed.     

Thermal characterization of longitudinal merging of turbulent spots

This study presents the thermal behavior of two young turbulent spots merging into a longitudinal direction on an isothermal flat plate for the local Reynolds number between 6.1 × 10⁴ and 1.3 × 10⁵ in a low freestream turbulence water tunnel having a turbulent intensity of 1.16%. The two turbulent spots are generated by water injection through a 1-mm-diameter hole in the perpendicular direction of the mainstream flow with a dimensionless separating time (Δτ) of 42.08, 84.16, and 126.24. Thermochromic liquid crystals are utilized mutually with an image processing technique to extract the spot characteristics qualitatively and quantitatively. The results demonstrate that the following turbulent spot directly causes an increase in the local Nusselt number and heat rate within the footprint of the merging spots. The relatively highest increase in this study occurs when Δτ = 84.16. The average Nusselt number and effectiveness characterize differently in the intersection zone, non-intersection zone of the leading spot, and non-intersection zone of the following spot. The results confirm that turbulent spots under the boundary layer transition augment the heat transfer rate to the level of full turbulence by not only their spot maturity but also the merging mechanism. Finally, the heat transfer mechanism is discussed and the predictive formulas for the Nusselt number and heat flux of the longitudinal merging of turbulent spots for Δτ from 0 to 126.24 are provided.     

Electron scattering from trans 1,3-butadiene molecule: cross-sections, oscillator strength and VUV photoabsorption cross-sections

Electron energy-loss spectra for the butadiene molecule were measured in the scattering angular range of 2.0° to 8.0°, in an energy-loss range from 2 to 50 eV, using 1000 eV incident electrons. The absolute generalized oscillator strength (GOS) and inelastic cross section have been determined for the \hbox{$\tilde{\rm X}^{1}$}?X1A _g  → 1¹B _u transition. The absolute elastic differential cross section was also determined spanning an angular range from 2.0° to 40.0°. From a small angle electron energy-loss spectrum, the optical oscillator distribution (photoabsorption spectrum) for the butadiene molecule was obtained in the 2 to 100 eV photon energy range. Accurate ab initio calculations have been performed, within the First Born Approximation, for generalized oscillator strength (GOS) and excitation energies for the \hbox{$\tilde{\rm X}^{1}$}?X1A _g  → 1¹B _u and \hbox{$\tilde{\rm X}^{1}$}?X1A _g  → 2¹A _g transitions. Our results emphasize the importance of using highly correlated wavefunctions and accurate methodologies in the calculation of the GOS for electron impact-induced electronic transitions in molecules.     

An Experimental Investigation of Liquid Jet Impingement and Single-Phase Spray Cooling Using Polyalphaolefin

Experiments on triangular and rectangular array jet impingement and single-phase spray cooling have been performed to determine the effect of both cooling techniques on heat transfer coefficient (h) and the coolant mass flux required for a given cooling load. Experiments were performed with circular orifices and nozzles for different H/D values from 1.5 to 26 and Reynolds number range of 219 to 837, which is quite lower than the ranges employed in widely used correlations. The coolant used was polyalphaolefin. The experiments simulated the boundary condition produced at the surface of the stator of a high power low-density generator or motor. For the custom fabricated orifices, commercial nozzles, and conditions used in this study, both cooling configurations showed enhancement of heat transfer coefficient as H/D increases to a certain limit after which it starts to decrease. The heat transfer coefficient always increases with Reynolds number. In keeping with previous studies, single-phase spray cooling technique can provide the same heat transfer coefficient as jets at a slightly lower mass flux, but with much higher-pressure head. Special Nu_d correlations that account for the range of parameters and coolant studied in this work are derived.     

Experimental Investigation of Natural Convection in Inclined Channel in Presence of Electrostatic Field

The influence of external electrohydrodynamic forces on natural convection from a flat plate has been studied. Temperature distribution in the aforementioned channel at α = 45°, 90°, and 150° angles in different 10-, 12-, and 15-kV electrostatic fields has been reviewed. Corona wind and the shape of the created vortex by electrohydrodynamics were detected. The results showed that heat transfer was enhanced by increasing the electrostatic field in all angles. In the presence of the field, enhancement of heat transfer of the plate at acute angles is better than for obtuse ones. The optimal angle in terms of heat transfer was 45°, noting heat transfer improvement by 61%.     

Experimental Study of Natural Convection from Electrically Heated Vertical Cylinders Immersed in Air

Abstract

An experimental study of laminar steady-state natural convection heat transfer from electrically heated vertical cylinders immersed in air has been undertaken. Three stainless steel (316 SS) test sections of different slenderness ratios were employed. Surface temperature profiles along the vertical cylinders were obtained using miniature thermocouples when the cylinders were heated with different power levels resulting in different total wall heat fluxes. After the mandatory corrections for the radiation heat fluxes were made, three correlation equations relating the local Nusselt number Nu_y with the local modified Rayleigh number Ra^* _y and the position-to-cylinder diameter y/d were developed. The correlation equations are valid for Ra^* _y ≤ 2 × 10¹².     

Experimental quantification of natural convective heat transfer within annulus space filled with a H2O-Cu nanofluid saturated porous medium. Application to electronics cooling

This experimental study deals with cooling electronics contained in a hemispherical cavity whose cupola is maintained isothermal, being its base inclined at an angle varying from 0° (horizontal disc with the cupola oriented upwards) to 135°. The active component is a dome centered on this base. The space between the differentially heated elements of the assembly is filled with a porous medium of high porosity saturated by a water–copper nanofluid whose volume fraction varies between 0% (pure water) and 7%. The Rayleigh number based on the radius of the cupola reaches high values up to 7.29 × 10¹⁰ given the important surface heat flux generated by the device during operation. The ratio between the thermal conductivity of the solid matrix and that of the base fluid ranges between 0 (interstitial volume without porous medium) and 41.4 corresponding to the intended applications. This experimental study done with an industrial prototype at scale 1 quantifies the natural convective heat transfer via the Nusselt number determined for many configurations obtained by varying the solid-fluid thermal conductivity ratio, the inclination angle, the Rayleigh number, and the volume fraction. The study clearly shows that the cooling performance of the Cu-H₂O nanofluid degrades with its age and the number of times it has been used. Analysis of the results reproducibility also proves the irreversibility of the performance. The measured values were compared with those obtained in a recent numerical study based on the volume control method. The observed deviations taking into account the experimental uncertainty margins validate the mathematical model implemented in the numerical approach.     

ESEEM study of the location of spin-polarized chlorophyll-quinone radical pair in membrane-oriented spinach photosystems I and II complexes

Light-induced spin-polarized radical pairs, P700⁺A₁ ^? in spinach photosystem (PS) I particles and P680⁺Q_A ^? in Zn-substituted PS II core complexes, in oriented membranes were studied by pulsed electron paramagnetic resonance (EPR). Based on the determined distance of 25.2 ± 0.2 Å between P700 and A₁, the angular dependence of the spin-polarized electron spin echo envelope modulation (ESEEM) spectra on the magnetic field suggests that the angle between $R_{P700 - {\rm A}_1 } $ , the radius-vector connecting P700 and A₁, and the membrane normaln was 24 ± 4° in PS I particles. Obtained angle and distance of P700-A₁ axis suggested Q_K side in the molecular geometry of cofactors presented in a recent X-ray crystallography of cyanobacterial PS I reaction center to be an active branch of electron transfer. The distance between P680 and Q_A was determined to be 27.4 ± 0.3 Å for a nonoriented PS II. The angle between $R_{P680 - Q_{\rm A} } $ , the radius-vector connecting P680 and Q_A, andn was determined to be 21 ± 5°. The angle of P680-Q_A axis was close to that of 20° of P870-Q_A axis reported in X-ray analysis of the purple bacterial reaction center crystal.