Mohamed S. El-Genk

Heat transfer of an impinging jet on a flat surface

Abstract Heat transfer between a uniformly heated flat plate and an impinging circular air jet was investigated experimentally to determine the values of the local and average Nusselt numbers, particularly for small values of Reynolds number and jet spacing. A heat transfer correlation was developed, which extends the existing database to Reynolds number and jet spacing values as low as 6000 and one jet diameter, respectively. Experimental results provided useful information of interest to potential industrial applications regarding the radius of the heat transfer area and jet spacing for maximizing the average Nusselt number.

Heat transfer and flow visualization experiments of swirling, multi-channel, and conventional impinging jets

Abstract Heat transfer and flow visualization experiments were conducted to investigate and compare the performance of swirling and multi-channel impinging jets with that of a conventional impinging jet (CIJ), having the same diameter at the same conditions. Swirling impinging jets (SIJs) employed a 25.4 mm long solid insert at the exit of housing tube to divert the air flow through four narrow channels along the surface of the insert, with the desired swirl angle (θ of 15, 30 and 45°). The multi-channel impinging jet (MCIJ) had same dimensions as SIJs, except that the narrow channels in the solid insert were vertical (θ = 0°). The local and surface average Nusselt numbers of MCIJ were generally much higher than those of CIJ. SIJs demonstrated large increases in both Nusselt numbers and significant enhancement in radial uniformity of heat transfer compared to MCIJ and CU; best results were for θ = 15° and jet spacing of 50.8 mm. Flow visualization experiments using smoke flow, smoke wires and water jet techniques revealed the mechanisms contributing to the higher and enhanced radial uniformity of heat transfer by SIJs. The smoke flow technique provided images of flow field between jet exit and impinged surface, while smoke wires showed details of flow field at and close to impinged surface. The water jet flow, seeded with tiny air bubbles as tracers, revealed details of flow field and induced mixing on the impinged surface.

Efficient segmented thermoelectric unicouples for space power applications

Abstract This paper compares the performance of SiGe (Si 0.8 Ge 0.2 ) and skutterudite segmented thermoelectric unicouples (STUs) at a hot side temperature of 973 K and cold side temperatures of 300, 573 and 673 K and for the same total length and cross sectional dimensions of the p-leg. The area of the n-leg and lengths of and the interfacial temperatures between segments of various materials in the STU legs are determined using a global optimization methodology. Results indicate that the STUs could potentially achieve peak efficiencies of 7.8% and 14.7% when operated at a cold side temperature of 573 K, typical of that in current radioisotope thermoelectric generators (RTGs), and 300 K, respectively. These efficiencies are 55% and 99% higher, respectively, than for SiGe at the same temperatures. However, due to the higher density of skutterudite, the electrical power densities corresponding to the peak efficiencies of the STUs are 39 and 109 W e /kg versus 92 and 232 W e /kg for SiGe at cold side temperatures of 573 K and 300 K, respectively. On the other hand, the heat inputs and heat rejection powers for the STUs are 42–55% and 39–53%, respectively, of those for SiGe at the peak efficiency and 70–75% and 67–77% of those of SiGe, respectively, at the peak electric power density. Therefore, when used in RTGs, STUs could halve the 238 PuO 2 fuel mass and the radiator area, while operating at >45% higher electrical power density (>7 W e /kg) than SiGe in current RTGs (∼5.5 W e /kg).

A heat pipe transient analysis model

Abstract A two-dimensional, heat pipe transient analysis model, ‘HPTAM’, is developed for simulating operations of fully-thawed heat pipes. The model is benchmarked using transient experimental data of a horizontal water heat pipe. The calculated steady-state water vapor and wall axial temperature profiles and the transient power throughput are in good agreement with measurements. Also presented and discussed are the calculated axial distributions of liquid and vapor pressures, effective radius of curvature of the liquid meniscus at the liquid-vapor interface, and liquid pooling and recession following step function heatup and cooldown transients of the water heat pipe.

Saturation boiling of HFE-7100 from a copper surface, simulating a microelectronic chip

Abstract Experiments are performed, which investigated the effect of inclination angle, θ , on saturation pool boiling of HFE-7100 dielectric liquid from a smooth, 10×10 mm copper surface, simulating a microelectronic chip. For θ ⩽90° and surface superheats, Δ T sat >20 K, nucleate boiling heat flux decreases with increased θ , but increases with θ for Δ T sat T sat >13 K, nucleate boiling heat flux decreases with increased inclination, but at lower surface superheats the trend is inconclusive. The developed nucleate boiling correlation is within ±10% of the data and the developed correlations for critical heat flux (CHF) and the surface superheat at CHF are within ±3% and ±8% of the data, respectively. Results show that CHF decreases slowly from 24.45 W/cm 2 at 0° to 21 W/cm 2 at 90°, then decreases fast with increased θ to 4.30 W/cm 2 at 180°. The surface superheat at CHF also decreases with θ , from 31.7 K at 0° to 19.9 K at 180°. Still photographs are recorded of pool boiling at different heat fluxes and θ =0°, 30°, 60°, 90, 120°, 150° and 180°. The measured average departure bubble diameter from the photographs taken at the lowest nucleate boiling heat flux of ∼0.5 W/cm 2 and θ =0° is 0.55±0.07 mm and the calculated departure frequency is ∼100 Hz.

Minimum thickness of a flowing down liquid film on a vertical surface

Abstract The minimum total energy (MTE) criteria for determining the minimum wetting rate and the minimum thickness of an isothermal, thin liquid film flowing down a vertical adiabatic surface are examined. An analytical expression of the profile of a stable liquid rivulet and of the two-dimensional velocity distribution in the rivulet developed using the Ritz method are incorporated into the MTE criterion to improve its predictions of the minimum wetting rate (MWR) and the corresponding minimum liquid film thickness (MLFT). The present predictions of the MWR and MLFT are in good agreement, to within ±10–20%, with experimental data for water and glycerol–water mixtures and with the values calculated from the MWR measurements, respectively. The accuracy of the present predictions is demonstrated for three conditions, namely: (a) incipient breakup of liquid film, (b) formation of a stable dry patch, and (c) rewetting of a dry surface. For the latter the advancing contact angle is substituted for the equilibrium contact angle in the developed analytical expressions for the MLFT and corresponding MWR.

Transient boiling from inclined and downward-facing surfaces in a saturated pool

Abstract Quenching experiments investigating transient pool boiling from the underside of inlcined and downward-facing flat surfaces in saturated water were performed. Inclination angles investgated are 0o (downward facing), 5°, 10°,15°,30°,45°90° vertical. While transitio boilition boiling heat flux and both qCHF and qmin, and the corresponding wall superheats increased as the inclanation angle nuleate boiling heat flux decreased. the values of qCHF and qmin and the corresponding are correlated as functions of the inclanation angle. In addition, the steady-stateqinCHF data of other investigators for saturated helium and nitrogen are correlated as function of inclination angle. these correlation are compared with that for water to determine the eefects of the type of the boiling liquid and heating method on qCHF

Thermal conductivity correlation for uranium nitride fuel between 10 and 1923 K

Abstract This research compiled and analyzed UN fuel thermal conductivity data covering the temperature range from 10 to 1923 K, and fuel density of 93% to 100% TD and, then developed a thermal conductivity correlation for 100% dense UN fuel: k(W/m K) = 1.37T 0.41 . This correlation predicts the thermal conductivity as a function of temperature within ± 10% of the data.

On colloidal particle sorption onto a stagnant air–water interface

Abstract This paper reviews reported experimental investigations of colloidal particle sorption onto a stagnant air–water interface and presents the results of some new work performed by the authors. A general consensus has emerged, based on the reported data, that particle sorption onto the interface is irreversible. Such irreversibility has been attributed to a capillary trapping caused by a net negative change in the interfacial energies, associated with transferring a fully immersed particle to the interface. The colloidal particle sorption onto an air–water interface depends on the energy barriers between sorbing particles and the interface and between particles in the bulk solution. The heights of these energy barriers, and the separation distances at which they occur, have been determined, based on the DLVO theory, after incorporating the effects of the solvation zone around particles and of the air–water interfacial region in calculating the van der Waals interaction energies.

Determination of operation envelopes for closed, two-phase thermosyphons

Abstract A concern in the design and operation of closed, two-phase therosyphons (CTPTs) is determining the initial filling ratio of the working fluid, as a function of CTPT dimensions, type and vapor temperature of working fluid and power throughput, to maximize performance, while avoiding potential dryout in the evaporator section. A one-dimensional, steady-state model is developed for determining the operation envelopes of CTPTs, in terms of the above parameters. The CTPT operation envelope is basically an enclosure with three boundaries. The lower boundary corresponds to when the liquid film thickness in the evaporator reaches a critical value—beyond it the liquid film could dry out—while the upper boundary corresponds to when the expanding liquid pool, due to boiling, fills the entire evaporator. The third and closing boundary corresponds to the counter-current flooding limit (CCFL) at the exit of evaporator. The correlation developed to calculate the expanded liquid pool height in the evaporator agrees with experimental data of acetone, ethanol and water to within ±8%. Also, the calculated upper and lower boundaries of the operation envelope for an ammonia CTPT are in excellent agreement with experimental data [1] . Calculations showed that increasing the CTPT diameter, evaporator length, or vapor temperature expands the operation envelope, while increasing either the condenser or the adiabatic section length only slightly changes the envelopes upper and lower boundaries.