Pool boiling of R-123/oil mixtures on enhanced tubes having different pore sizes

The effect of enhanced geometry (pore diameter, gap width) is investigated on the pool boiling of R-123/oil mixture for the enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 5% oil concentration, the degradation is 26–49% for T_sat = 4.4 °C. The degradation increases 50–67% for T_sat = 26.7 °C. The heat transfer degradation is significant even with small amount of oil (20–38% degradation at 1% oil concentration for T_sat = 4.4 °C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. The maximum degradation is observed for d_p = 0.20 mm tube at T_sat = 4.4 °C, and d_p = 0.23 mm tube at T_sat = 26.7 °C. The minimum degradation is observed for d_p = 0.27 mm tube for both saturation temperatures. It appears that the oil removal is facilitated for the larger pore diameter (along with larger gap) tube. The highest heat transfer coefficient with oil is obtained for d_p = 0.23 mm tube, which yielded the highest heat transfer coefficient for pure R-123. The optimum tube significantly (more than 3 times) outperforms the smooth tube even with oil. The heat transfer degradation increases as the heat flux decreases.     

Airside performance of herringbone wavy fin-and-tube heat exchangers – data with larger diameter tube

This study examines the airside performance of the wavy fin-and-tube heat exchangers having a larger diameter tube (D_c = 16.59 mm) with the tube row ranging from 1 to 16. It is found that the effect of tube row on the heat transfer performance is quite significant, and the heat transfer performance deteriorates with the rise of tube row. The performance drop is especially pronounced at the low Reynolds number region. Actually more than 85% drop of heat transfer performance is seen for F_p ～ 1.7 mm as the row number is increased from 1 to 16. Upon the influence of tube row on the frictional performance, an unexpected row dependence of the friction factor is encountered. The effect of fin pitch on the airside performance is comparatively small for N = 1 or N = 2. However, a notable drop of heat transfer performance is seen when the number of tube row is increased, and normally higher heat transfer and frictional performance is associated with that of the larger fin pitch.     

The effect of porous media particle size on forced convection from a circular cylinder without assuming local thermal equilibrium between phases

A detail numerical analysis of the effect of particle diameter of a packed bed of spherical particles on forced convection about an embedded circular cylinder is presented. This parametric study focusses on the two-phase energy (LTNE—local thermal non-equilibrium) model, which does not assume local thermal equilibrium (LTE) between the solid medium and the fluid. The investigation is performed for a cylinder-to-particle diameter ratio D_cy/d_p = 10–100, at a wide ranges of Reynolds number Re_D = 1–250 and solid-to-fluid thermal conductivity ratio k_r = 0.01–1000. A comparison of predictions from the LTNE and LTE energy models is also made. This paper quantifies the influence of the key non-dimensional parameters on the heat transfer rate. It is also shown that although the presence of the porous materials around the heated cylinder enhances the overall heat transfer and increases the pressure drop in the bed compared to an empty channel, using a porous medium with large particle diameters increases considerably this enhancement in heat transfer and decreases significantly the unfavorable pressure drop.     

Effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions

This study investigated the effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions. A total of 10 samples were tested with associated fin thickness (δ_f) of 0.115 mm and 0.25 mm, respectively. For a heat exchanger with two rows (N = 2) and fin pitch F_p of 1.41 mm, the effect of fin thickness on the heat transfer coefficient is more pronounced. The heat transfer coefficients for δ_f = 0.25 mm is about 5–50% higher than those for δ_f = 0.115 mm whereas the pressure drop for δ_f = 0.25 mm is about 5–20% higher. The unexpected difference in heat transfer coefficient subject to fin thickness is attributable to better interactions between the directed main flow and the swirled flow caused by the condensate droplet for δ_f = 0.25 mm. The maximum difference in heat transfer coefficients for N = 2 and F_p = 2.54 mm subject to the influence of fin thickness is reduced to about 20%, and there is no difference in heat transfer coefficient when the frontal velocity is above 3 m/s. For N ⩾ 4 and F_p = 2.54 mm, the influence of fin thickness on the heat transfer coefficients diminishes considerably. This is because of the presence of tube row, and the unsteady/vortex shedding feature at the down stream of wavy channel. Based on the present test results, a correlation is proposed to describe the air-side performance for wavy fin configurations, the mean deviations of the proposed heat transfer and friction correlations are 7.9% and 7.7%, respectively.     

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.     

Influence of the flashing phenomenon on the boiling curve of refrigerant R134a in minichannels

The results of experimental investigations of heat transfer during the flow of R134a in a minichannel are presented here. The experimental investigations were conducted using a minichannel with a total length of 500 mm and 1.68 mm internal diameter. The heated length of the minichannel was 200 mm, the total mass flow rate of the refrigerant (wρ) = 200–450 kg/m² s, the inlet subcooling ΔT_s = 5–15 K, and the heat flux density q = 1.7–60.3 kW/m². The results of experimental investigations are presented as a boiling curve. The phenomenon known as the zero boiling crisis and the influence of the flashing phenomenon on the boiling curve show the importance of these elements on heat transfer in single- and two-phase systems.     

The particle thermal conductivity influence of nanofluids on thermal performance of the microtubes

The paper presents the numerical analysis on microchannel laminar heat transfer and fluid flow of nanofluids in order to evaluate the suitable thermal conductivity of the nanoparticles that results in superior thermal performances compared to the base fluid. The diameter ratio of the micro-tube was D_i/D_o = 0.3/0.5 mm with a tube length L = 100 mm in order to avoid the heat dissipation effect. The heat transfer rate was fixed to Q = 2 W. The water based Al₂O₃, TiO₂ and Cu nanofluids were considered with various volume concentrations ϕ = 1,3 and 5% and two diameters of the particles d_p = 13 nm and 36 nm. The analysis is based on a fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results reveal that only the nanofluids with particles having very high thermal conductivity (λ_Cu = 401 W/m K) are justified for using in microcooling systems. Moreover, the analysis is sensitive to both the comparison criteria (Re or Π) and heat transfer parameters (h_ave or t_max).     

Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube with small pipe inserts

Heat transfer performance and pressure drop tests were performed on a circular tube with small pipe inserts. These inserts with different spacer lengths (S = 100, 142.9 and 200 mm) and arc radii (R = 5, 10 and 15 mm) were tested at Reynolds numbers between 4000 and 18,000. Tap water was used as working fluid. The use of pipe inserts allowed for a high heat transfer coefficient with relatively low flow resistance. The Nusselt number and friction factor increase with the decrease in spacer length. Optimal results were obtained for S = 100 mm (R = 10 mm). Heat transfer rates and friction factors were enhanced by 2.09–2.67 and 1.59–1.85 times, respectively, to those in the plain tube. Performance evaluation criterion (PEC) values were approximately 1.79–2.17. The Nusselt number and friction factor increase with the decrease in arc radius. Small pipe inserts with R = 5 mm and S = 100 mm show maximal heat transfer rates of 2.61–3.33 and friction factors of 1.6–1.8 times those of the empty tube. The PEC values were 2.23–2.7. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their unique structure.     

The effect of different inlet geometries on laminar flow combined convection heat transfer inside a horizontal circular pipe

The effect of different inlet geometries on laminar air flow combined convection heat transfer inside a horizontal circular pipe has been experimentally investigated for Reynolds number range of 400–1600, and the Grashof number range from 3.12 × 10⁵ to 1.72 × 10⁶. The experimental setup consists of an aluminum circular pipe as a heated section with 30 mm inside diameter and 900 mm heated length (L/D = 30) with different inlet geometries. A wall boundary heating condition of a uniform heat flux was imposed. The inlet configurations used in this paper are calming sections having the same inside diameter as the heated pipe but with variable lengths of L_calm. = 600 mm (L/D = 20), L_calm. = 1200 mm (L/D = 40), L_calm. = 1800 mm (L/D = 60), L_calm. = 2400 mm (L/D = 80), sharp-edged and bell-mouth. It was found that the surface temperature values for calming section length corresponding to (L/D = 80) were higher than other inlet geometries due to the lower mass flow rate and higher flow resistance. It was also observed that the Nusselt number values for bell-mouth inlet geometry were higher than other inlet geometries due to the differences in the average temperatures and densities of the air. The average heat transfer results were correlated with an empirical correlation in terms of dependent parameters of Grashof, Prandtl and Reynolds numbers. The proposed correlation was compared with available literature and it shows reasonable agreement.     

Experimental study of forced convection in asymmetrically heated sintered porous channels with/without periodic baffles

This study experimentally determined the local and average heat transfer characteristics in asymmetrically heated sintered porous channels with metallic baffles. The fluid medium was air. Measurements on the test specimen of four modes, without baffles (A), with periodic baffles on the top portion (B), with periodic baffles on the bottom portion (C) and with staggered periodic baffles on both sides (D), are performed. The effect of the average bead diameter was also examined (d = 0.704 and 1.163 mm). The data indicated that, the wall temperatures measured at baffles attached to the heated wall were slightly lower than those nearby, especially at high Reynolds numbers. In modes B and D, the heat transfer in the inlet region was weaker than that in modes A and C. Additionally, the heat transfer by forced convection in all modes increased as the bead diameter decreased. The effect of the bead diameter became stronger as the Reynolds number was increased. At Re > 2000, heat transfer was greatest in mode B and least in mode D, in which the heat transfer was even poorer than that without baffles. For a Re of around 1000, mode D was associated with an excellent heat transfer. In such a case, heat transfer enhancement was around 20 ∼ 30% in mode D, around 10 ∼ 20% in mode B and around 0 ∼ 12% in mode C.     

Experimental investigation of mixed convection heat transfer for thermally developing flow in a horizontal circular cylinder

An experimental investigation is presented on mixed (free and forced) convection to study the local and average heat transfer for hydrodynamically fully developed, thermally developing and thermally fully developed laminar air flow in a horizontal circular cylinder. The experimental setup consists of aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30), is subjected to a constant wall heat flux boundary condition. The investigation covers Reynolds number range from 400 to 1600, the heat flux varied from 60 W/m² to 400 W/m² and with cylinder inclination angle of θ = 0° (horizontal). The hydrodynamically fully developed condition is achieved by using an aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 180 cm (L/D = 60), another one with length of 240 cm (L/D = 80) and two short calming sections with lengths 60 cm (L/D = 20), 120 cm (L/D = 40). The surface temperature variation along the cylinder surface, the local and average Nusselt number variation with the dimensionless axial distance Z⁺ were presented. For all entrance sections, it was found an increase in the Nusselt number values as the heat flux increases. It was concluded that the free convection effects tended to decrease the heat transfer results at low Re while to increase the heat transfer results for high Re. The combined convection regime could be bounded by a suitable selection of Re number ranges and the heat flux ranges. The obtained Richardson numbers (Ri) range varied approximately from 0.13 to 7.125. The average Nusselt numbers were correlated with the (Rayleigh numbers/Reynolds numbers). The proposed correlation has been compared with available literature and showed satisfactory agreement.     

Airside performance of fin-and-tube heat exchangers in dehumidifying conditions – Data with larger diameter

This study presents the airside performance of the fin-and-tube heat exchangers having plain fin geometry with a larger diameter tube (D_c = 15.88 mm) under dehumidifying condition. A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch are made and tested. It is found that the effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row. However, there is a unique characteristic of fin pitch at a shallow tube row, the heat transfer performance is first increased at a wider pitch but a further increase of fin pitch lead to a falloff of heat transfer performance due to interactions amid flow development and bypass flow. The influence of tube row on the airside performance is rather small for both heat transfer and frictional characteristics at a fin pitch of 2.1 mm and when the Reynolds number is less than 4000. A slight deviation of this effect is encountered when fin pitch is increased to 2.54 mm or 3.1 mm due to condensate adhered phenomena.     

Average heat transfer rates measured in two different temperature ranges for magnetic convection of horizontal water layer heated from below

The average heat transfer rates of gravitational and magnetic convection of water heated from below and cooled from above are measured for two cases of cold wall temperature θ_c at 10 °C and 30 °C. The height of the cylindrical enclosure is 2 mm with 40 mm in diameter. The magnetic field is imposed in a vertical direction to increase or decrease 29% of the gravitational acceleration in a bore space of a super-conducting magnet of 10 T at the solenoid center. The group of data at θ_c = 30 °C gives a better agreement with the classical heat transfer rate of Silveston than that at θ_c = 10 °C. This is probably due to the almost constant value in the volumetric magnetic susceptibility of water at about 10 °C.     

Bénard convection from a circular cylinder in a packed bed

Bénard convection around a circular heated cylinder embedded in a packed bed of spheres is studied numerically. The Forchheimer–Brinkman–extended Darcy momentum model with the Local Thermal Non-Equilibrium energy model is used in the mathematical formulation for the porous layer. The governing parameters considered are the Rayleigh number (10³ ≤ Ra ≤ 5 × 10⁷) and the thermal conductivity ratio (0.1 ≤ k_r ≤ 10,000). The structural properties of the packed bed are kept constant as: cylinder-to-particle diameter ratio D/d = 20 and porosity ε = 0.5, while the Prandtl number is fixed at Pr = 0.71. It is found that the presence of the porous medium suppresses significantly the strong free convection produced in the empty enclosure, and reduces considerably the high intensity of the pair of vortices generated behind the cylinder. Also, the results show that the porous medium can play the role of insulator or enhancer of heat transfer from the heat source, depending mainly on their thermal conductivities regardless of the Rayleigh number.     

Natural convective heat transfer of heated packed beds

Natural convection heat transfer of heated packed bed was investigated. Experiments were performed for a single heated sphere buried in unheated packed beds varying its locations and for packed beds with all heated spheres varying the heights of packed beds from 0.02 m to 0.26 m. Mass transfer experiments using a copper electroplating system were performed based upon the analogy between heat and mass transfer. The diameter of sphere was 0.006 m, which corresponds to Ra_d of 1.8 × 10⁷. For the single heated sphere cases, the measured results agreed well with the existing natural convection heat transfer correlations for packed beds and even with those for a single sphere in an open channel. For all heated sphere cases, the average heat transfers decrease with increasing packed bed heights.     

An experimental investigation of the effect of longitudinal fin orientation on heat transfer in membrane water wall tubes in a circulating fluidized bed

Experiments were conducted in a cold model circulating fluidized bed having riser cross-sectional area of 100 mm × 100 mm, height of 4.8 m, bed temperature of 75 °C and superficial velocity of 8 m s^?1. Local sand having average diameter of 231 μm was used as bed material. The experiments were conducted for three tube configurations: membrane tube, membrane tube with a longitudinal fin at the tube crest and membrane tube with two longitudinal fins at 45° on both sides of the tube crest. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes and the heat is mainly transferred in the combination portion of tube and membrane fins. In addition, the membrane tube has the highest heat transfer coefficient.     

Cooling of two smooth cylinders in row by a slot jet of air with low turbulence

According to the current literature on the cooling of two cylinders in row, by a uniform flow of air, the first cylinder is always a heat transfer promoter versus the second one. The aim of the present paper is to summarize the state of art of the literature on the cooling of two cylinders in row by a slot jet of air. Additional experiments are carried on in order to investigate the possible application of jet cooling to heat transfer apparatuses, including electronics, in order to study the positions of the two cylinders in row which realize the same heat transfer on each cylinder. In the experiments a slot jet of air with low turbulence is employed with a slot height, S, equal to the impinged cylinder diameter, D, i.e. D/S = 1.0. The first cylinder is set at two distances H from the slot exit, H/S = 4 and 6, while the distance of the second cylinder from the first one, L, is variable from L/S = 2–11. The Reynolds number, Re, defined with the cylinder diameter D, spans in the range Re = 11,000–22,200. If the first cylinder is set at the dimensionless distance from the slot exit which realizes the maximum mean heat transfer on the first cylinder, i.e. H/S = 6, the second one has generally a lower mean Nusselt number. The only exception is when the second cylinder is set at the dimensionless distance L/S = 4 and the Reynolds number is at the maximum value experimented, i.e. Re = 22,200. If the first cylinder is set at the dimensionless distance H/S = 4 the mean Nusselt number on the second cylinder is greater if its distance from the first one is in the range L/S = 3.5–7 for Re = 14,300–22,200. The first cylinder acts as a heat transfer promoter, as happens in uniform flow, only for Re = 22,200.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Combined convection heat transfer for thermally developing aiding flow in an inclined circular cylinder with constant heat flux

Experiments have been conducted to study the local and average heat transfer by mixed convection for hydrodynamically fully developed, thermally developing and thermally fully developed laminar air flow in an inclined circular cylinder. The experimental setup consists of aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30), is subjected to a constant wall heat flux boundary condition. The investigation covers Reynolds number range from 400 to 1600, heat flux is varied from 70 W/m² to 400 W/m² and cylinder angles of inclination including 30°, 45° and 60°. The hydrodynamically fully developed condition has been achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 180 cm (L/D = 60), another one with length of 240 cm (L/D = 80) and two short calming sections with lengths of 60 cm (L/D = 20), 120 cm (L/D = 40). The results present the surface temperature distribution along the cylinder length, the local and average Nusselt number distribution with the dimensionless axial distance Z⁺. For all entrance sections, the results showed an increase in the Nusselt number values as the heat flux increases and as the angle of cylinder inclination moves from θ = 60° inclined cylinder to θ = 0° horizontal cylinder. The mixed convection regime has been bounded by the convenient selection of Re number range and the heat flux range, so that the obtained Richardson numbers (Ri) is varied approximately from 0.13 to 7.125. The average Nusselt numbers have been correlated with the (Rayleigh numbers/Reynolds numbers) in empirical correlations.     

Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel

Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.15 ? w_in ? 0.16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 × 10⁷ < Ra¹ < 8 × 10⁸ and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H_f/H = 0.25 to H_f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8–9 mm and optimum fin spacing depends on the value of Ra¹.