Heat transfer in a turbulent slot jet flow impinging on concave surfaces

An experimental and numerical study is conducted to investigate turbulent slot jet impingement cooling characteristics on concave plates with varying surface curvature. Air is used as the impingement coolant. In the experimental work, a slot nozzle specially designed with a sixth degree polynomial in order to provide a uniform exit velocity profile was used. The experiments were carried out for the jet Reynolds numbers in the range of 3423 ≤ Re ≤ 9485, the dimensionless nozzle-to-surface distance range of 1 ≤ H/W ≤ 14 for dimensionless values of the curvature of impinging surfaces in the range of R/L = 0.5, 0.725, and 1.3 and a flat impingement surface. Constant heat flux was applied on the plates. Numerical computations were performed using the k-ε turbulence model with enhanced wall functions. For the ranges of the governing parameters studied, the stagnation, and local and average Nusselt numbers have been obtained both experimentally and numerically. The numerical results showed a reasonable agreement with the experimental data.     

Effect of flow geometry parameters on transient heat transfer for turbulent flow in a circular tube with baffle inserts

The effect of the flow geometry parameters on transient forced convection heat transfer for turbulent flow in a circular tube with baffle inserts has been investigated. The characteristic parameters of the tubes are pitch to tube inlet diameter ratio H/D = 1, 2 and 3, baffle orientation angle β = 45°, 90° and 180°. Air, Prandtl number of which is 0.71, was used as working fluid, while stainless steel was considered as pipe and baffle material. During the experiments, different geometrical parameters such as the baffle spacing H and the baffle orientation angle β were varied. Totally, nine types of baffle inserted tube were used. The general empirical equations of time averaged Nusselt number and time averaged pressure drop were derived as a function of Reynolds number corresponding to the baffle geometry parameters of pitch to diameter ratio H/D, baffle orientation angle β, ratio of smooth to baffled cross-section area S_o/S_a and ratio of tube length to baffle spacing L/H were derived for transient flow conditions. The proposed empirical correlations were considered to be applicable within the range of Reynolds number 3000 ⩽ Re ⩽ 20,000 for the case of constant heat flux.     

Experiments on impingement heat transfer with film extraction flow on the leading edge of rotating blades

A transient liquid crystal experiment was performed to study the heat transfer characteristic of impingement cooling with outflow film on the leading edge of turbine blades under rotating conditions. In the experiments, the angles between the jet direction and rotating shaft were 0°, 30°, and 45°, respectively. The impinging jet Reynolds number, based on the diameter of the impingement hole, ranged from 2000 to 12,000. The rotation number Ro (Ωd/u) ranged from 0 to 0.278. The relative impingement distance was fixed at 2. The results showed that, due to the effect of rotation, the spreading rate of the jet flow was enhanced and the heat transfer was weakened for all Reynolds numbers. For the condition of Re = 4000 and Ro = 0.139 with corresponding angles θ = 0°, 30°, 45°, the Nusselt number of the stagnation point decreased by 33%, 30%, and 35%, respectively, compared to the stationary results. Furthermore, for the corresponding angles θ = 30° and 45°, the location of the stagnation point is offset 0.6d (jet impingement hole diameter) and 0.9d down, respectively, when Ro = 0.139. The average Nusselt numbers on the suction surface and the pressure surface both decreased with increased rotating speed. Moreover, the reduction of the average Nusselt number on the pressure surface was larger than that on the suction surface. At Ro = 0.139, the average Nusselt number on the suction surface decreased less than 10% for all three angles, while on the pressure surface, the decrease was almost 20% compared to the result for Ro = 0.     

Experimental investigation of natural convection from vertical and horizontal helicoidal pipes in HVAC applications

Natural convection from uniformly heated helicoidal pipes oriented vertically and horizontally is experimentally studied. Four helicoidal pipes of different parameters are presented. The effects of pitch to pipe diameter ratio, coil diameter to pipe diameter ratio and length to pipe diameter ratio on the average heat transfer coefficient are found. The experiments covered a range of Rayleigh number based on tube diameter from 1.5 × 10³ to 1.1 × 10⁵. The results showed that the overall average Nusselt number, Nu_m, increases with the increase in pitch to pipe diameter ratio, coil diameter to pipe diameter ratio and length to pipe diameter ratio for vertical helicoidal pipes. For horizontal helicoidal pipes, the overall average Nusselt number, Nu_m, increases with the increase in pitch to pipe diameter ratio and length to pipe diameter ratio, but it decreases with the increase in coil diameter to pipe diameter ratio. New correlations are presented, and they can be used in HVAC applications.     

DNS study of pulsed film cooling for enhanced cooling effectiveness

Direct Numerical Simulation (DNS) of a pulsed film-cooling jet is presented to examine if pulsations of the coolant jet can enhance film-cooling effectiveness. Calculations are performed for a cylindrical jet inclined at 30°. The jet pulsation is defined by the duty cycle (DC) and the Strouhal number (St), both of which are varied in this study. Baseline calculations are done for a steady blowing ratio of 1.5. Both frequency and duty cycle are observed to influence the cooling effectiveness. For a peak blowing ratio (M) of 1.5, pulsing with a St = 0.32 and DC = 0.5 is shown to reduce jet blow-off and improve centerline and spanwise-averaged effectiveness over the steady M = 1.5 case.     

Experimental investigations on heat transfer and frictional characteristics of a turbulator roughened solar air heater duct

An experimental investigation has been carried out to study the heat transfer coefficient and friction factor by using artificial roughness in the form of specially prepared inverted U-shaped turbulators on the absorber surface of an air heater duct. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters.The experiments encompassed the Reynolds number range from 3800 to 18000; ratio of turbulator height to duct hydraulic mean diameter is varied from, e/D_h = 0.0186 to 0.03986 (D_h = 37.63 mm and e = 0.7 to 1.5 mm) and turbulator pitch to height ratio is varied from, p/e = 6.67 to 57.14 (p = 10 to 40 mm). The angle of attack of flow on turbulators, α = 90° kept constant during the whole experimentation. The heat transfer and friction factor data obtained is compared with the data obtained from smooth duct under similar geometrical and flow conditions. As compared to the smooth duct, the turbulator roughened duct enhances the heat transfer and friction factor by 2.82 and 3.72 times, respectively. The correlations have been developed for area averaged Nusselt number and friction factor for turbulator roughened duct.     

An experimental investigation of steam ejector refrigeration system powered by extra low temperature heat source

A steam ejector refrigeration system is a low capital cost solution for utilizing industrial waste heat or solar energy. When the heat source temperature is lower than 80 °C, the utilization of the thermal energy from such a low-temperature heat source can be a considerable challenge. In this investigation, an experimental prototype for the steam ejector refrigeration system was designed and manufactured, which can operate using extra low-temperature heat source below 80 °C. The effects of the operation temperature, the nozzle exit position (NXP) and the diameter of the constant area section on the working performance of the steam ejector were investigated at generating temperatures ranging from 40 °C to 70 °C. Three ejectors with a same de Laval nozzle for the primary nozzle and three different constant-area sections were designed and fabricated. The experimental results show that a steam ejector can function for a certain configuration size of the steam ejector with a generating temperature ranging from 40 °C to 70 °C and an evaporating temperature of 10 °C. For a given NXP, the system COP and cooling capacity of the steam ejector decreased until inoperative as the diameter of the constant area section reduced. The results of this investigation provided a good solution for the refrigeration application of the steam ejector refrigeration system powered by an extra low-temperature heat source.     

Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers

Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solution sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems.In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capacity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side.Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated.In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U_overall varied from 970 W/m² °C (170 Btu/h ft² °F) to 2270 W/m² °C (400 Btu/h ft² °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry.     

Rewetting and maximum surface heat flux during quenching of hot surface by round water jet impingement

The transient cooling of hot stainless steel surface of 0.25 mm thickness is done with round water jet impingement. Initially, the surface was heated up to the temperature of 800 °C before the water was injected through straight tube type nozzle of 2.5 mm diameter and 250 mm length. During impingement cooling, the surface temperature was measured up to 12 mm radial distance away from the stagnation point. The jet exit to surface spacing, z/d, and jet Reynolds number, Re, varied in the range of 4–16 and 5000–24,000 respectively. The surface rewetting and transient heat flux of the test-surface was studied for these operating parameters.During impingement cooling process the initial rewetting occurred at stagnation region with the lowest wetting delay period. In fact, the rewetting temperature, rewetting velocity and the maximum heat flux reduced for extreme spatial location. However, the wetting delay increased significantly for the locations away from the stagnation point. The surface rewetting and transient heat flux were increased with the rise in jet Reynolds number, resulting in the enhancement in rewetting temperature, rewetting velocity and reduced wetting delay. The maximum heat flux was obtained for 4–6 mm radial location. The effect of jet exit to surface spacing on the rewetting parameters is found to be marginal. A correlation has been developed which predicted the maximum heat flux within an error band of ±10%.     

Boiling curves in relation to quenching of a high temperature moving surface with liquid jet impingement

Experimental investigations have been conducted for quenching of a hot rotating cylinder with initial temperature of about 500–600 °C by a subcooled planar water jet. An original experimental device allowing the estimation of the local boiling curves in the case of a static surface and of a moving surface has been designed. Heat fluxes were measured on both side of the axis of the jet until a reduced distance x/l of 18, in a range of subcooling from 10 to 83 K, for a jet velocity from 0.8 to 1.2 m/s and for a velocity flow-surface ratio (u_S/u_j) from 0.5 to 1.25. In the case of static surface, the measurements confirmed the existence of a “shoulder of flux” in the stagnation zone of the jet. In the case of a moving surface, the maximum of heat transfer (for a given regime) is moving during the cooling time from downstream (film boiling regime) to upstream (forced convection).     

Optimum temperatures in a shell and tube condenser with respect to exergy

This paper focuses on evaluation of the optimum cooling water temperature during condensation of saturated water vapor within a shell and tube condenser, through minimization of exergy destruction. First, the relevant exergy destruction is mathematically derived and expressed as a function of operating temperatures and mass flow rates of both vapor and coolant. The optimization problem is defined subject to condensation of the entire vapor mass flow and it is solved based on the sequential quadratic programming (SQP) method. The optimization results are obtained at two different condensation temperatures of 46 °C and 54 °C for an industrial condenser. As the upstream steam mass flow rates increase, the optimal inlet cooling water temperature and exergy efficiency decrease, whereas exergy destruction increases. However, the results are higher for optimum values at a condensation temperature of 54 °C, compared to those when the condensation temperature is 46 °C. For example, when the steam mass flow rate is 1 kg/s and the condensation temperature increases from 46 °C to 54 °C, the optimal upstream coolant temperature increases from 16.78 °C to 25.17 °C. Also, assuming an ambient temperature of 15 °C, the exergy destruction decreases from 172.5 kW to 164.6 kW. A linear dependence of exergy efficiency on dimensionless temperature is described in terms of the ratio of the temperature difference between the inlet cooling water and the environment, to the temperature difference between condensation and environment.     

Activation of raw pitch coke with alkali hydroxide to prepare high performance carbon for electric double layer capacitor

Powder of raw pitch coke was activated with alkali hydroxides at 500–900 °C to prepare carbon electrode of high capacitance for electric double layer capacitor (EDLC). KOH provided very high surface area of 2320 m²/g at 800 °C, while NaOH did moderate surface area of 1000 m²/g at 650–750 °C. High surface area provided by KOH led to a high capacitance per weight of 39 F/g. However, its capacitance per volume was as low as 16 F/ml. Although the coke of moderate surface area activated with NaOH showed a similar capacitance per weight, its capacity per volume was as high as 28 F/ml because of its high density. Adequate porosity must be selectively introduced by NaOH activation to the coke to obtain moderate surface area. Much smaller expansion of layers in the present needle type coke activated by NaOH than that by KOH is indicative for the higher density of the former activated coke.     

Influence of natural convection on the melting of ice block surrounded by water on all sides

The ice block at initial temperature T_is = 0 °C is fixed at the center of a long, prismatic enclosure with isothermal vertical walls and insulated horizontal walls. The enclosure is completely filled with water at initial temperature T_il = 0 °C. Six numerical simulations were performed by varying vertical wall temperatures from T_W = 2 to 12 °C (range of Rayleigh number from 4.22 × 10⁶ to 2.28 × 10⁷). In the case of T_W > 8 °C the ice melts faster from above and for T_W < 8 °C from below. In the case of T_W = 8 °C, two vortices are separated by nearly vertical 4 °C isotherm and the average Nusselt number remains constant during the convection dominated regime.     

Film cooling at hypersonic Mach numbers using forward facing array of micro-jets

Experimental investigations are carried out in the IISc hypersonic shock tunnel on film cooling effectiveness of a single jet (diameter 2 mm and 0.9 mm), and an array forward facing of micro-jets (diameter 300 μm each) of same effective area (corresponding to the respective single jet). The single jet and the corresponding micro-jets are injected from the stagnation zone of a blunt cone model (58° apex angle and nose radius of 35 mm). Nitrogen and Helium are injected as coolant gases. Experiments are performed at freestream Mach number 5.9, at 0° angle of attack, with a stagnation enthalpy of 1.84 MJ/kg, with and without injections. The ratios of the jet stagnation pressure to the freestream pitot pressure used in the present study are 1.2 and 1.45. Up to 50% reduction in surface heat transfer rate was observed with the array of micro-jets, compared to that of the respective single jet with nitrogen as the coolant, while the corresponding reduction was up to 37% for helium injection, with the schlieren flow visualizations showing no major change in the shock standoff distance, and thus no major changes in other aerodynamic aspects such as drag.     

Performance evaluation of mixed convection in an inclined square channel with uniform temperature walls

Numerical analyses were performed for the effect of inclined angle on the mixing flow in a square channel with uniform temperature walls (T_w = 30 °C) and inlet temperature (T₀ = 10 °C). Three-dimensional governing equations were solved numerically for Re = 100, Pr = 0.72 and various inclined angles (from ?90° to 90°). Three-dimensional behavior of fluid in a channel was examined for each angle. Thermal performance was evaluated using the relationship between Nusselt number ratio and pressure loss ratio with and without buoyancy induced flow as a parameter of inclined angles. High heat transfer and low pressure loss region was from ?15° to ?60° in thermal performance using mean Nusselt number ratio.     

Continuous esterification for biodiesel production from palm fatty acid distillate using economical process

An overflow system for continuous esterification of palm fatty acid distillate (PFAD) using an economical process was developed using a continuous stirred tank reactor (CSTR). Continuous production compared to batch production at the same condition had higher product purity. The optimum condition for the esterification process was a 8.8:1:0.05 molar ratio of methanol to PFAD to sulfuric acid catalyst, 60 min of residence time at 75 °C under its own pressure. The free fatty acid (FFA) content in the PFAD was reduced from 93 to less than 1.5%wt by optimum esterification. The esterified product had to be neutralized with 10.24%wt of 3 M sodium hydroxide in water solution at a reaction temperature of 80 °C for 20 min to reduce the residual FFA and glycerides. The components and properties of fatty acid methyl ester (FAME) could meet the standard requirements for biodiesel fuel. Eventually the production costs were calculated to disclose its commercialization.     

Biodiesel production by esterification of palm fatty acid distillate

Production of fatty acid methyl ester (FAME) from palm fatty acid distillate (PFAD) having high free fatty acids (FFA) was investigated in this work. Batch esterifications of PFAD were carried out to study the influence of: including reaction temperatures of 70–100 °C, molar ratios of methanol to PFAD of 0.4:1–12:1, quantity of catalysts of 0–5.502% (wt of sulfuric acid/wt of PFAD) and reaction times of 15–240 min. The optimum condition for the continuous esterification process (CSTR) was molar ratio of methanol to PFAD at 8:1 with 1.834 wt% of H₂SO₄ at 70 °C under its own pressure with a retention time of 60 min. The amount of FFA was reduced from 93 wt% to less than 2 wt% at the end of the esterification process. The FAME was purified by neutralization with 3 M sodium hydroxide in water solution at a reaction temperature of 80 °C for 15 min followed by transesterification process with 0.396 M sodium hydroxide in methanol solution at a reaction temperature of 65 °C for 15 min. The final FAME product met with the Thai biodiesel quality standard, and ASTM D6751-02.     

Heat flow visualization for natural convection in rhombic enclosures due to isothermal and non-isothermal heating at the bottom wall

Analysis has been carried out for the energy distribution and thermal mixing in steady laminar natural convective flow through the rhombic enclosures with various inclination angles, φ for various industrial applications. Simulations are carried out for various regimes of Prandtl (Pr) and Rayleigh (Ra) numbers. Dimensionless streamfunctions and heatfunctions are used to visualize the flow and energy distribution, respectively. Multiple flow circulations are observed at Pr = 0.015 and 0.7 for all φs at Ra = 10⁵. On the other hand, two asymmetric flow circulation cells are found to occupy the entire cavity for φ = 75° at higher Pr (Pr = 7.2 and 1000) and Ra (Ra = 10⁵). Heatlines are found to be parallel circular arcs connecting the cold and hot walls for the conduction dominant heat transfer at Ra = 10³. The enhanced convective heat transfer is explained with dense heatlines and convective loop of heatlines at Ra = 10⁵. Heatlines clearly demonstrate that the left wall receives heat from the bottom wall as heatlines directly connect both the walls whereas the convective heat circulation cells play lead role to distribute the heat along the right wall, especially for smaller φs. On the other hand, the heat flow is evenly distributed to both side walls at higher φs via convection as well as direct conductive transport. Significant convective heat transfer from the bottom hot wall to the left cold wall occurs for φ = 30° cavity whereas the heat transfer to the right cold wall is maximum for φ = 75° irrespective of Pr. Average Nusselt number studies also show that φ = 30° cavity gives maximum heat transfer rate from the bottom to left wall irrespective of Pr in isothermal heating case. On the other hand, enhanced thermal mixing occurs at φ = 75° for both isothermal and non-isothermal heating strategies except at Pr = 0.015 in isothermal heating case.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having multi v-shaped with gap rib as artificial roughness

This paper presents the results of an experimental investigation of heat transfer and friction in the flow of air in rectangular ducts having multi v-shaped rib with gap roughness on one broad wall. The investigation encompassed Reynolds number (Re) from 2000 to 20,000, relative gap distance (G_d/L_v) values of 0.24–0.80, relative gap width (g/e) values of 0.5–1.5, relative roughness height (e/D) values of 0.022–0.043, relative roughness pitch (P/e) values of 6–12, relative roughness width ratio (W/w) values of 1–10, angle of attack (α) range of 30°–75°. The optimum values of geometrical parameters of roughness have been obtained and discussed. For Nusselt number (Nu), the maximum enhancement of the order of 6.74 times of the corresponding value of the smooth duct has been obtained, however the friction factor (f) has also been seen to increase by 6.37 times of that of the smooth duct. The rib parameters corresponding to maximum increase in Nu and f were G_d/L_v = 0.69, g/e = 1.0, e/D = 0.043, P/e = 8, W/w = 6 and α = 60°. Based on the experimental data, correlations for Nu and f have been developed as function of roughness parameters of multi v-shaped with gap rib and flow Reynolds number.     

Structural evolution of Eucalyptus tar pitch-based carbons during carbonization

Wood tar pitches are generated as by-products by the charcoal manufacturing industry. They have a macromolecular structure constituted mainly by phenolic, guaiacylic, and siringylic units common to lignin. Due to their characteristics, biopitches are been investigated as precursors of carbon materials such as carbon fibers, bioelectrodes and activated carbons. In the present work the structural evolution of Eucalyptus tar pitches under carbonization is investigated, which is important for the improvement of planning and control of pitch processing and end-product properties during carbon material production. The studies involve X-ray diffraction and infrared analyses, besides helium density, BET surface area and BJH pore volume measurements. The results showed that the conversion of pitch into carbon basically involves three steps: (1) Up to around 600 °C the material has an highly disordered structure, being the release of aliphatic side chains and volatiles the main events taking place. (2) Between 600 °C and 800 °C, condensation of aromatic rings occurs to form bi-dimensional hexagonal networks so that micro- and mesoporosity are developed. The 800 °C-coke is constituted by two phases: one highly disordered and another more crystalline. (3) Over 800 °C, both phases are gradually ordered. As defects are gradually removed, surface area and porosity decrease, approaching zero for the 2100 °C-coke.