Heat transfer and friction in rectangular solar air heater duct having spherical and inclined rib protrusions as roughness on absorber plate

In this work, an experimental investigation on heat and fluid flow characteristics of artificially roughened solar air heater duct has been carried out. The roughness and operating parameters covered a range of Reynolds number (Re) from 2000 to 20000, relative roughness pitch (P/e) from 15 to 30, relative rib length (r/g) from 0.4 to 1.0, and relative rib pitch (P_r/P) from 0.2 to 0.8. Other parameters, i.e., relative roughness height (e/D), angle of attack (α), and relative roughness gap are kept constant. Results show that the considerable enhancement in Nusselt number and Thermo-hydraulic performance has been obtained with an increase in friction factor using roughened surface.     

Flow and Heat Transfer in Rough Micro Steel Tubes

Abstract

This article investigates the flow and heat transfer characteristics in micro steel tubes with inner diameters of 168 μm, 399 μm and relative roughness of 3.5% and 2.7%, respectively, by measuring the friction factors and the Nusselt number from laminar state to transitional state. Experiments show that the experimental Nusselt numbers are less than those predicted by the classical laminar correlation due to the effect of the variation of the thermophysical properties with temperature when Reynolds number is low. As the Reynolds number is higher than 800, the experimental Nusselt number are 25–50% higher than the predictions of the classical laminar and transitional correlations due to the effects of the roughness and the entrance length. The transition from laminar to turbulent flow occurs at the Reynolds number of 1,100–1,500.     

Effect of roughness elements on thermal and thermohydraulic performance of double pass solar air heater duct having discrete multi V-shaped and staggered rib roughness on both sides of the absorber plate

In this work, an experimental setup consisting of a double pass solar air heater duct provided with discrete multi V-shaped and staggered rib as artificial roughness on both sides of the absorber plate has been designed and fabricated. The parameters of interest investigated experimentally cover a wide range of Reynolds number (Re) from 2000 to 20000, relative staggered rib pitch (p’/p) from 0.2 to 0.8, relative staggered rib size (r/e) from 1 to 4, and relative roughness width (W/w) from 5 to 8. Experimental data related to heat transfer, pressure loss, and thermohydraulic performance has been determined. Substantial improvement has been obtained with some penalty of friction losses.     

Kinetic and mechanistic study on the thermal isomerization of ocimene in the liquid phase

The rate of thermal isomerization of ocimene in the liquid phase has been investigated in the range 90–150 °C. The rate constant for the disappearance of ocimene may be expressed by k = 1. 3 × 10¹⁰e^{– 11994. 2/T}(min^{– 1}), from which we can infer that the activation energy is 99.7 kJ mol^–1 and the pre‐exponential factor is 1.3 × 10¹⁰ min^–1. The half‐life for the disappearance of ocimene may be expressed by t_1/2 = 5. 2 × 10^{– 11}e^{11994. 2/T}(min). The conclusion has been supported by the study results that the ocimene is safe when temperature is below 100 °C. A discussion of the mechanism concerning the conversion is included. Copyright © 2011 John Wiley & Sons, Ltd.     

Flow visualization and local mass transfer studies for turbulent flow in a wind tunnel with chamfered ribs

The paper presents results of flow visualization and mass transfer studies for fully developed turbulent flow of air in a square section wind tunnel with repeated chamfered rib roughness on the bottom of the tunnel (rib head chamfer angles ϕ of -15°, 0°, and 15°; relative roughness pitchp/e = 3, 5, 7.5, and 10). Direct video recordings of flow patterns were made using a simple technique of particles visualization. For the positively chamfered closely spaced ribs (p/e <- 5) vigorous vortex shedding has been seen compared to the square or negatively chamfered ribs, which is found to be a function of the Reynolds number. For the widely spaced ribs, the study shows flow separation at the ribs and reattachment in the inter-rib region. Local mass transfer studies, based on the variation in colour of cobaltous chloride solution impregnated paper due to evaporation of water, showed a significant improvement in mass transfer rate in the recirculating region in the wake of ribs with the change in the chamfer angle from -15° to 15°. The positively chamfered 15° ribs are found to be better than square section ribs atp/e <- 7.5. The performance of negatively chamfered ribs is found to be poor compared to other ribs irrespective of the relative roughness pitch.     

FRICTION AND CONVECTION STUDIES OF R-134a IN MICROCHANNELS WITHIN THE TRANSITION AND TURBULENT FLOW REGIMES

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.     

Heat transfer and friction characteristics of solar air heater duct having protruded roughness geometry on absorber plate

Protruded roughness geometry is generally considered to be simple, an innovative and economic technique for improving thermal performance of solar air heater. Experimental investigation has been carried to study the effect of different shapes, position, and height of protrusions on heat transfer and friction characteristics of solar air heater duct for the range of parameters. Correlations for Nusselt number and friction factor have been brought out from the experimental data which can be further used for evaluating performance of solar air heater having the considered roughness geometry.     

Untersuchungen zum Existenzbereich einer BOLTZMANN-Verteilung im Argonbogenplasma

Using as plasma source a wall stabilized argon arc working within a restricted parameter range (inner tube diameter = 7 mm, pressure = 30–120 Torr, current = 5-20 A)) the existence of a Boltzmann equilibrium between spectral energy levels is checked by comparing measured occupation number densities of higher excited levels (N_{m, exp}) with the corresponding number densities calculated under the assumption of Boltzmann equilibrium (N_{m, calc}). The methods for determination of the quantities N_{m, exp'} T_g (2300–5405°K), T_e (7170–9950°K) and N_e (0.33 – 2.4 × 10¹⁵ cm^?3) needed for this comparison are described. It can be shown within the limit of experimental error that a Boltzmann equilibrium exists at least for electron densities of N_e > 3 · 10¹⁴ cm^?3. The problem of energy balance of that type of arcs used in these experiments is discussed.     

Observation of e ± in the inelastic e + A interaction

Results of observation and analysis of e ^± appearing in the interaction of 32-GeV positrons with nuclear emulsion have been reported. The angular distributions of particles have been measured in the range of 0°–180°. The energy of particles has been measured in the angular range of 145°–180° and values of the Bjorken variable x _B have been calculated. The distribution in x _B corresponds to values smaller than unity at Q ² > 7.0 (GeV/s)². This means that e ^± with energies lower than 250 MeV cannot be attributed to multinucleon correlations.     

Enhancing the thermal performance of AL2O3/DI water nanofluids in micro-fin tube equipped with straight and left-right twisted tapes in turbulent flow regime

This article communicates the thermal performance, heat transfer rate, and friction factor of Al₂O₃/DI water nanofluids at different concentrations in a micro-finned tube with tube helical inserts for different twist ratios. The thermal performance, heat transfer coefficient, and friction of the present study is also compared with a plain tube for validation. From the study, it is identified that the micro-finned tube with tube insert performance is higher as compared with a plain tube. Similarly, an empirical relation for Nusselt number (Nu) and friction factor (f) is estimated for straight twisted tube and left-right combination. The deviation between experimental and theoretical values for left-right twist and straight twist is found as 3 and 7% for Nusselt number and 7 and 9% for friction factor, respectively. Similarly, while analyzing the thermal performance, it was found that the maximum performance achieved was with a micro-fin tube with left-right twist with nanofluid concentration of 0.2%.     

Inner–outer interactions in rough-wall turbulence

Here we revisit the inner–outer interaction model (IOIM) of Marusic et al. (Science, vol. 329, 2010, pp. 193–196) that enables the prediction of statistics of the fluctuating streamwise velocity in the inner region of wall-bounded turbulent flows from a large-scale velocity signature measured in the outer region of the flow. The model is characterised by two empirically observed inner–outer interactions: superposition of energy from outer region large-scale motions; and amplitude modulation by these large-scale motions of a small-scale ‘universal’ signal (u^*), which in smooth-wall flows is Reynolds number invariant. In the present study, the inner–outer interactions in rough-wall turbulent boundary layers are examined within the framework of the IOIM. Simultaneous two-point hot-wire anemometry measurements enable quantification, via the model parameters, of the strengths of superposition and amplitude modulation effects in a rough-wall flow, and these are compared to a smooth-wall flow. It is shown that the present rough-wall significantly reduces the effects of superposition, while increasing the amplitude modulation effect. The former is true even in flows that exhibit outer region similarity. Using the model parameters obtained from the two-point measurements, predictions of inner region streamwise velocity statistics and spectra are compared to measurements over a range of friction and roughness Reynolds numbers. These results indicate that the u^* signal does depend on roughness Reynolds number (k⁺_s), but is robust to changes in friction Reynolds number (δ⁺). Additionally, the superposition strength is shown to be relatively independent of both roughness and friction Reynolds number. The implications of the present results on the suitability of the IOIM as a predictive tool in rough-wall turbulence are discussed.     

Effect of Swirl Generator Inserted Into a Tube on Exergy Transfer: Decaying Flow

This study experimentally investigates the effect of conical injector type swirl generator inserts on heat and exergy transfer in a uniform heat flux tube for turbulence regime. One hundred twenty watts of heat are applied by coiling a flexible silicone heater on the tube (copper pipe) with an inside diameter of 62 mm and length of 1,200 mm. The conical injector type swirl generator has a conical shape, which is similar to a funnel, with angle (α) of 30°, 45°, and 60°. Circular holes are drilled on the conical injector type swirl generator as three different cross-sectional areas (A_h). The total areas (A_t = N.A_h) of the holes on the conical injector type swirl generator equal each other. Because of this, the three different holes numbers (N) are produced. Flow directors having three different angles (β = 30°, 60°, and 90°) to radial direction are attached to every one of the holes. This study is a typical example for decaying flow. Reynolds number (Re) was varied from 10,000 to 35,000, so the flow was considered as only a turbulent regime. All experiments were conducted with air accordingly; Prandtl number was approximately fixed at 0.71. The local and average heat transfer Nusselt number (Nu_h) and exergy transfer Nusselt number (Nu_e) are calculated and discussed in this article. It is found that the Nu_h increases with an increase in Reynolds number, director angle (β), and director diameter (d) but with a decrease in the conical injector type swirl generator angle (α). However, Nu_e decreases depending on the same independent parameters.     

Experimental Investigation of Subcooled Vertical Upward Flow Boiling in a Narrow Rectangular Channel

Accurate models for the onset of nucleate boiling, density of active nucleation sites (N_a), bubble departure size (D_d), and departure frequency (f_d) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G = 122–657 kg/m²s), inlet subcooling (ΔT_sub = 4.7–33.3?C), and heat flux (q″ = 1.7–28.9 W/cm²). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, N_a, D_d, and f_d for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12°C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications.     

A high‐level theoretical study into the atmospheric phase hydration,bond dissociation enthalpies,and acidity of aldehydes

CBS‐Q//B3, G4(MP2), and G4 composite method calculations were used to estimate atmospheric phase standard state (298.15 K, 1 atm) free energies of hydration (Δ_hydrG°_(g)), hydration equilibrium constants (log K_hydr,(g)), bond dissociation enthalpies (BDEs), and enthalpies (Δ_dH°_(g)) and free energies (Δ_dG°_(g)) of aldehydic proton acid dissociation for various substituted aldehydes with electron withdrawing and electron releasing groups. Good quality log K_hydr,(g) correlations with the Swain–Lupton resonance effect parameters R and R⁺ were found, allowing extension of the model to predict log K_hydr,(g) values for 487 substituted aldehydes having available R‐values and 108 substituted aldehydes having available R⁺ values. Good correlations were also found between experimental aqueous phase hydration equilibrium constants (log K_hydr,(aq)) and summative R/R⁺ values for peripheral substituents on a range of carbonyl derivatives (aldehydes, ketones, esters, and amides), suggesting that the structure–reactivity modeling approach can be extended to include all possible combinations of R₁C(O)R₂ carbonyl substitution in both gas and aqueous systems. Computationally derived BDEs and Δ_dH°_(g)/Δ_dG°_(g) were in good agreement with the limited experimental and theoretical datasets. BDEs did not generally correlate with any of the Hammett substituent constants or Swain–Lupton parameters considered. Gas phase acidities exhibited high correlation coefficients with Hammett inductive substituent constants (σ_I) and field effect parameters (F), allowing these to be employed as surrogates for estimating the gas phase aldehydic proton acidities of a larger potential compound range. The resulting models will be of use in predicting the environmental behavior for a broad range of environmentally relevant compounds containing carbonyl functionalities. Copyright © 2016 John Wiley & Sons, Ltd.     

An analysis of piling-up or sinking-in behaviour of elastic–plastic materials under a sharp indentation

Finite-element simulation has been carried out to investigate the piling-up or sinking-in behaviour of elastic–plastic strain-hardening materials under a sharp indentation. An empirical model is proposed to relate the contact area to the material parameter E/σ_y and the experimental parameter h _e/h _max. Materials with E/σ_y<89.07 in the limit of vanishing friction and E/σ_y<150.7 for a friction coefficient μ?=?0.2 may show only sinking-in. Other materials may show either piling-up or sinking-in depending on the h _e/h _max ratio or strain-hardening exponent. There is no universal h _e/h _max ratio that can be used to predict the piling-up or sinking-in behaviour of materials. Rather, the critical h _e/h _max ratio or strain-hardening exponent for no piling-up or sinking-in is a function of E/σ_y. The piling-up or sinking-in behaviour of materials is found to be closely related to fully plastic deformation. The influence of friction on the amount of piling-up or sinking-in has also been considered in this study.     

Ab initio calculations for propyne and the hydrogen bonded complex NH H C C CH 3 3

The hydrogen-bonded cluster NH₃ …H—C≡C—CH₃ has been investigated by means of the coupled electron pair approximation, making use of a basis set of 198 contracted Gaussian-type orbitals. The calculated equilibrium structure is r _1^e (N—H) = 1?0127 Å, α_e(∠HN…H) = 112?32°, R _1^e (N…H) = 2?3593 Å, r _2^e (acetylenic C—H) = 1?0690 Å, R _2^e (C≡C) = 1?2078 Å, R _3^e (C—C) = 1?4711 Å, r _3^e (C—H) = 1?0894 Å and β_e(∠CCH) = 110?50°. The recommended equilibrium dissociation energy is D _e = 12?4±0?5 kJ mol^-1 and the calculated equilibrium dipole moment is μ_e = – 1?468 D, with the positive end of the dipole at the ammonia protons. Harmonic wavenumbers and absolute infrared intensities for the totally symmetric modes are calculated. Compared with free propyne the acetylenic CH stretching vibration experiences a bathochromic shift of 93 cm^-1 and an intensity enhancement by a factor of 5?5.     

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.     

Rotational Analysis of the 1–4 Band of the B 2Σ+–X 2Σ+ System of 14C16O+

ABSTRACT

High-resolution emission spectrum of the 1–4 band of the B ²Σ⁺–X ²Σ⁺ transition of ¹⁴C¹⁶O⁺ was observed for the first time by conventional emission spectroscopy. The band spectrum was excited in a water-cooled Geissler lamp filled with commercial gaseous carbon monoxide enriched in about 80% of the radiocarbon ¹⁴C. A rotational analysis has been carried out and obtained molecular constants have been merged with previously published data for the B ²Σ⁺–A ²Π_i and A ²Π_i–X ²Σ⁺ transitions. The principal equilibrium constants for the ground X ²Σ⁺ state obtained from this work are ω_e = 2121.7726(98), ω_e x _e = 13.9055(27), B _e = 1.815290(30), α_e = 1.6594(33) × 10^?2, and γ_e = ? 0.377(73) × 10^?4 cm^?1. Also, presently known experimental equilibrium molecular constants of the X ²Σ⁺ states of the CO⁺ isotopic molecules are summarized and isotopic dependence of the B _e and ω _e constants is discussed.