Pressure drop and heat transfer during two-phase flow vaporization of propane in horizontal smooth minichannels

This study examined the two-phase flow boiling pressure drop and heat transfer for propane, as a long term alternative refrigerant, in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5–20 kW m^?2, mass fluxes ranging from 50–400 kg m^?2 s^?1, saturation temperatures of 10, 5 and 0 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and lengths of 1000 mm and 2000 mm, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Because the study on evaporation with propane in minichannels was limited, new correlations of pressure drop and boiling heat transfer coefficient were developed in this present study.     

Effects of biofouling on air-side heat transfer and pressure drop for finned tube heat exchangers

Experimental investigations on the effects of biofouling on air-side heat transfer and pressure drop for three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were performed. Artificial accelerated method of microorganism growth on the fin surface was used for simulating the biofouled finned tube heat exchangers. Experimental results indicate that the effects of biofouling on the air-side heat transfer coefficient decreases 7.2% at 2.0 m/s when the biofouled area ratio is 10%, while it decreases 15.9% at 2.0 m/s when the biofouled area ratio is 60%, and biofouling causes a 21.8% ～ 41.3% increase in pressure drop when the air velocity is between 0.5 and 2.0 m/s. The increase of inlet air velocity is helpful to improve the long-term performance of finned tube heat exchanger. Biofouling makes the hydrophilic coating failure, and the condensation water easily converges on the fin surface where biofouling grows.     

Effect of the geometry of flattened tube on the thermal performance of a high temperature generator

The present study numerically investigated the effect of the geometry of flattened tube on the thermal performance of a high temperature generator (HTG) with the pre-mixed surface flame burner of the double effect LiBr–water absorption system. The heat transfer tubes of the HTG were consisted with a set of circular and flattened tubes in series. FLUENT, as a commercial code, was applied for estimating the thermal performance of the HTG. Key parameters were the aspect ratio of flattened tubes, rib transversal length, and the rib pitch ratio on the flattened tube of the HTG. The maximum heat transfer rate of the HTG was obtained at the aspect ratio of 6.8 for the flattened tube. The heat transfer rate for the flattened tube was increased by 4.2% as the rib transversal length was increased from 2 mm to 3 mm. The heat transfer rate of the flattened tube with the rib pitch ratio of 15.3 was higher by the maximum 5.8% than that without rib. The heat transfer rate of the HTG with the rib of the rib transversal length of 3 mm and the rib pitch ratio of 15.3 was higher by 3.4% than that without the rib. It led that the exhaust gas temperature of the HTG with the rib was lower by 23 °C than that without the rib.     

A heat transfer correlation for natural draft wire-and-tube condensers

A dimensionless correlation to estimate the heat transfer coefficient between the external surfaces of natural draft wire-and-tube condensers and the surrounding air is proposed in this study. The experiments were carried out using a purpose-built testing apparatus that reproduces the actual operating conditions of such heat exchangers. A test section to vary the condenser position in relation to the adjacent surfaces was also developed and mounted within a controlled environmental chamber. The correlation was developed based on a total of 72 experimental data points obtained with 24 different condenser samples with tube outer diameter  ranging from 4.8 to 6.2 mm, number of tube rows from 13 to 25, and number of wire pairs from 10 to 90. The tests were performed with an average temperature difference between the condenser surface and the ambient air ranging from 3.6 to 36.1 °C. The proposed correlation predicts 90% of heat transfer data within an error band of ±10%. Comparisons with other correlations available in the open literature are also reported.     

Experimental investigation of evaporation heat transfer coefficient and pressure drop of R-410A in a multiport mini-channel

The evaporation heat transfer coefficient and pressure drop of R-410A flowing through a horizontal aluminium rectangular multiport mini-channel having 3.48 mm hydraulic diameter are experimentally investigated. The test runs are performed at mass flux ranging between 200 and 400 kg/m² s. The heat fluxes are between 5 and 14.25 kW/m² and the saturation temperatures range between 10 and 30 °C. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of the imposed wall heat flux, mass flux, vapour quality, and saturation temperature on the evaporation heat transfer and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the existing correlation. New correlations for the evaporation heat transfer coefficient and pressure drop of R-410A flowing through a multiport mini-channel are proposed for practical applications.     

Effect of twisted tape insert on heat transfer and pressure drop in horizontal evaporators for the flow of R-134a

An experimental study has been carried out on the heat transfer enhancement and pressure drop characteristics in presence of twisted tape inserts, during flow boiling of R-134a, inside a horizontal evaporator. The test-evaporator was an electrically heated 1260 mm long copper tube with 7.5 mm inside diameter. The experiments were performed for plain flow and four tubes with twisted tapes of 6, 9, 12 and 15 twist ratios and four refrigerant mass velocities of 54, 85, 114 and 136 kg/s m² for each tape. It has been found that the twisted tape inserts enhance the heat transfer coefficient on relatively higher pressure drop penalty, in comparison to that for the plain flow.     

Failure analysis of heat exchanger tubes

Primary waste heat exchanger tubes of material ASTM A213 grade T11 failed after operation of only three and a half months. The heat exchanger was of the bayonet type with boiler water inside the tubes and secondary reformer outlet process gas at the shell side. The heat exchanger environment was rich in hydrogen, carbon monoxide and nitrogen. The temperature of the process gases was 960 °C and the heat exchanger was producing steam at a temperature of 306 °C and a pressure of 1500 psig. The failed, used and new heat exchanger tubes were subjected to stereo/optical microscopy, chemical analysis and hardness testing. The cause of the failure was thoroughly investigated using optical/scanning electron microscope equipped with energy dispersive spectrometer. The study revealed that the material was exposed to thermal cycling and excessive local heating. The same was also confirmed by simulated experimentation. These conditions lead to thermal fatigue of the material with consequent failure.     

Failure analysis of AISI 321 tubes of heat exchanger

The present study is about the failures of tubes from a tube shell heat exchange in one of the compression stages used to cool and purify the gas produced in an off-shore platform. Conventional hydrostatic tests did not detect leaking in the heat exchanger, but these tests were conducted at room temperature, i.e., very distant to the service temperature (163 °C). However, tightness tests with helium gas have detected leaking. Some tubes were taken off for analysis and the liquid penetrating test detected cracks in crevices formed between the tube and the tube sheet. The cracks were found to be typical stress corrosion cracks with branched pattern. A non-destructive test with Eddy current probes was developed to give high percentage of detection. The work was conclusive about the main cause of failures. Due to the increase of chloride content of the water processed to concentrations higher than 1000 mg/L over the years, the substitution of AISI 321 tubes by superduplex steel is recommended.     

Assessment on the use of common correlations to predict the mass-flow rate of carbon dioxide through capillary tubes in transcritical cycles

Capillary tubes are extensively used in small refrigeration and air-conditioning systems with synthetic refrigerants and hydrocarbons. For CO₂ transcritical applications, it has been shown that the capillary tube demonstrates an intrinsic capability of adjusting the upper pressure close to the optimal value in response to changes of gas-cooler heat sink temperature. The CO₂ flow rate through four capillary tubes of various lengths, diameters and materials was measured in a test rig. Each capillary tube was tested with inlet pressure varying from 7.5 MPa to 11 MPa and inlet temperature from 20 °C to 40 °C. Outlet pressure varied from 1.5 MPa to 3 MPa. The experimental results were validated against different numerical and approximate analytical solutions of the capillary tube equations. These models give good predictions only if the friction factor of the capillary tube is calculated accounting for its dependence on the tube roughness.     

Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

Research on heat treatment of TiBw/Ti6Al4V composites tubes

Heat treatment with different parameters were performed on the hot-hydrostatically extruded and swaged 3.5 vol.% TiBw/Ti6Al4V composites tubes. The results indicate that the primary α phase volume fraction decreases and transformed β phase correspondingly increases with increasing solution temperatures. The α + β phases will grow into coarse α phases when the aging temperature is higher than 600 °C. The hardness and ultimate tensile strength of the as-swaged TiBw/Ti6Al4V composite tubes increase with increasing quenching temperatures from 900 to 990 °C, while they decrease with increasing aging temperatures from 550 to 650 °C. A superior combination of ultimate tensile strength (1388 MPa) and elongation (6.1%) has been obtained by quenching at 960 °C and aging at 550 °C for 6 h. High temperature tensile tests at 400–600 °C show that the dominant failure modes at high temperatures also differ from those at room temperature.     

The effect of heat treatment temperature on the microstructure and magnetic properties of Ba2Co2Fe12O22 (Co2Y) prepared by sol–gel method

Ferroxplana type hexagonal ferrite with composition Ba₂Co₂Fe₁₂O₂₂ (Co₂Y) was prepared by polymeric sol–gel method from the aqueous solution of their corresponding metal salts. All the samples were characterized by using X-ray diffraction and scanning electron microscopic technique. The formation and the microstructure of the Ba₂Co₂Fe₁₂O₂₂ was studied as a function of heat treatment temperature and it was observed that well defined hexagonal plate-like fine particles with particle sizes between 130 and 400 nm were formed at relatively lower temperature (900 °C) as compared to conventional solid state reaction method (1100 °C). Magnetic properties of the samples were also studied as a function of heat treatment temperature and it showed large saturation magnetization (M_s) ranging from 30.9 to 47.6 emu/g and a wide range of coercivity (130∼1566 Oe) depending on the heat treatment temperature. The very high saturation magnetization at relatively lower temperature (say 800 °C) arises due to the presence of several high magnetic impurities, such as BaFe₁₂O₁₉ and CoFe₂O₄. The saturation magnetization increases with increasing heat treatment temperature above 900 °C, while the coercivity showed a reversed order.     

Two-phase heat transfer and pressure drop of LNG during saturated flow boiling in a horizontal tube

Two-phase heat transfer and pressure drop of LNG (liquefied natural gas) have been measured in a horizontal smooth tube with an inner diameter of 8 mm. The experiments were conducted at inlet pressures from 0.3 to 0.7 MPa with a heat flux of 8–36 kW m⁻², and mass flux of 49.2–201.8 kg m⁻² s⁻¹. The effect of vapor quality, inlet pressure, heat flux and mass flux on the heat transfer characteristic are discussed. The comparisons of the experimental data with the predicted value by existing correlations are analyzed. Zou et al. (2010) correlation shows the best accuracy with 24.1% RMS deviation among them. Moreover four frictional pressure drop methods are also chosen to compare with the experimental database.     

Bonding strength of heat treated compressed Eastern redcedar wood

The objective of this study was to evaluate effect of heat treatment and compression on some properties of Eastern redcedar (Juniperus virginiana) including bonding strength, hardness and surface quality. Specimens were exposed to three temperature levels of 120 °C, 160 °C and 190 °C for 6 h before they were compressed using 2.5 MPa pressure for 5 min. Polyvinyl acetate (PVAc) bonded specimens showed 23.6% reduction in their shear strength when they were exposed to a temperature of 120 °C. Such strength reduction values were 44.4% and 64.1% for the specimens exposed to temperature levels of 160 °C and 190 °C, respectively. The lowest average Janka hardness value of 214.08 kg was determined for the samples exposed to a temperature of 190 °C while those treated with a temperature of 120 °C had the highest hardness value of 397.73 kg. It appears that combination of heat treatment and compression enchanced overall surface quality of the samples in the form of their roughness determined using stylus type equipment.     

Evaluation of hardness and surface quality of different wood species as function of heat treatment

The objective of this study was to evaluate the effect of heat treatment on surface roughness and hardness of four wood species, namely black alder (Alnus glutinosa L.), red oak (Quercus falcata Michx.), Southern pine (Pinus taeda L.) and yellow poplar (Liriodendron tulipifera). Samples were exposed to heat treatment schedules having two temperature and exposure levels of 120 °C and 190 °C for 3 and 6 h, respectively. Average hardness value of red oak samples exposed to a temperature of 190 °C for 6 h was 41.7% lower than that recorded before the heat treatment. Temperature of 190 °C produced 7.9% lower hardness values for black alder with the increased exposure time from 3 h to 6 h. No significant differences were found between same type of Southern pine and yellow poplar specimens before and after the heat treatment in terms of their hardness values. Among the four species considered in this study red oak having the most porous anatomical structure showed the roughest surface. An improvement in surface quality (R_a) with 7.46% with extending exposure time from 3 h to 6 h at the temperature level of 190 °C was noticed. However all four types of wood species kept in the oven at 190° for 6 h presented smoother surface quality. It was found that increased temperature from 120 °C to 190 °C for both exposure times showed significant differences from the surface quality of nontreated samples at 95% confidence level. The anatomical structure of samples was also observed by scanning electron microscope (SEM) and some damage of the cell wall was determined due to heat treatment. The findings of this study demonstrated that heat treatment resulted in adverse effect on hardness characteristics of the samples. It appears that strength losses can be limited through alternative modified heat treatment techniques. On the other hand, surface quality of the samples from all species was enhanced as a result of heat treatment. Therefore such heat treatment would be considered to improve surface quality of the sample for furniture applications where smooth surfaces are ideal adding potential value on wood material to be used more effectively in furniture manufacturing.     

Combustion characteristics of the heat pellet prepared from the Fe powders obtained by spray pyrolysis

Fe powders for thermal batteries were prepared by reduction of iron oxide powders obtained by spray pyrolysis. The iron oxide powders prepared by spray pyrolysis had fine size, spherical shape and high surface area. The morphologies of the Fe powders were affected by the preparation temperatures of the iron oxide powders. The Fe powders obtained from the iron oxide powders prepared by spray pyrolysis at 900 and 1000 °C had slightly aggregated structure of the primary powders with several microns sizes. The powders had pure Fe phases at reducing temperatures between 600 and 800 °C. The heat pellets with diameter of 18.2 mm were prepared using Fe powders and potassium perchlorate (KClO₄). The porosity of the prepared heat pellet was about 40%. The break strength of the heat pellet was 0.9 kgf. The ignition sensitivity of the heat pellet was 4 W. The maximum burn rate of the heat pellet obtained from the Fe powders were 8.6 cm s^?1.     

Effect of heat treatment on microstructure,hardness and rollability of V55Ti30Ni15 alloy membranes

The effect of heat treatment on the microstructure, hardness and rollability of V₅₅Ti₃₀Ni₁₅ alloy membranes has been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine NiTi particles precipitate from the supersaturated V-matrix solid solution at temperatures above 600 °C, increase in quantity until 800 °C, then dissolve back into the V-matrix with a further increase in temperature up to 950 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 950 °C. In the present study, a comparison has been made between the rollability of the as-cast and the heat treated state selected for deformation at different rolling temperatures. The percent reduction in thickness of the heat-treated alloy (800 °C/18 h) has been found to be up to 30% higher than that of the as-cast alloy, even at room temperature (cold rolling).     

Effect of heat treatment on mechanical properties of low alloy steel foams

Effect of heat treatment on compressive properties of low alloy steel foams (Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C) having porosities in the range of 47.4–71.5% with irregular pore shape, produced by the space holder-water leaching technique in powder metallurgy, was investigated. Low alloy steel powders were mixed with different amounts of space holder (carbamide), and then compacted at 200 MPa. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were sintered at 1200 °C for 60 min in hydrogen atmosphere. Sintered compacts were heat treated by austenitizing at 850 °C for 30 min and then quenched at 70 °C in oil and tempered at 210 °C for 60 min. In this porosity range, compressive yield strengths of as-sintered and heat treated specimens were 28–122 MPa and 18–168 MPa, respectively. The resultant Young’s moduli of the as-sintered and heat treated specimens were 0.68–3.12 GPa and 0.47–3.47 GPa, respectively. The heat treatment enhanced the Young’s modulus and compressive yield strength of the foams having porosities in the range of 47.4–62.3%, as a consequence of matrix strengthening. However, the compressive yield stress and Young’s modulus of the heat treated foam having 71.5% porosity were lower than that of the as-sintered foam’s, as a result of cracks in the structure. The results were discussed in light of the structural findings.     

Experimental study of the influence of cold heat exchanger geometry on the performance of a co-axial pulse tube cooler

Improving the performance of the pulse tube cooler is one of the important objectives of the current studies. Besides the phase shifters and regenerators, heat exchangers also play an important role in determining the system efficiency and cooling capacity. A series of experiments on a 10 W @ 77 K class co-axial type pulse tube cooler with different cold heat exchanger geometries are presented in this paper. The cold heat exchangers are made from a copper block with radial slots, cut through using electrical discharge machining. Different slot widths varying from 0.12 mm to 0.4 mm and different slot numbers varying from around 20–60 are investigated, while the length of cold heat exchangers are kept the same. The cold heat exchanger geometry is classified into three groups, namely, constant heat transfer area, constant porosity and constant slot width. The study reveals that a large channel width of 0.4 mm (about ten times the thermal penetration depth of helium gas at 77 K, 100 Hz and 3.5 MPa) shows poor performance, the other results show complicated interaction effects between slot width and slot number. These systematic comparison experiments provide a useful reference for selecting a cold heat exchanger geometry in a practical cooler.     

Effects of heat treatments on the microstructure and mechanical properties of Rene 80

Effects of heat treatments on room temperature mechanical properties and stress-rupture properties of Rene 80 have been investigated. The microstructures were analyzed by optical microscope and scanning electron microscope after each step of heat treatments. With the decrease of aging temperature, the average size of γ′ phase decreases, but the volume fraction of γ′ phase increases. The lower aging temperature suppresses the growing of the coarse γ′ particles, but facilitates the growth of the fine γ′ particles. After the optimum heat treatment, the ultimate tensile strength and yield strength are respectively higher than 1040 MPa and 950 MPa, the stress-rupture life at 871 °C/310 MPa is higher than 170 h with excellent ductility. The improved tensile strength and stress-rupture life are primarily due to the increased volume fraction of γ′ phase. The borides precipitate at grain boundaries at about 913 °C. The primary MC is found to decompose into M₆C at about 873 °C and M₂₃C₆ at 840–873 °C at grain boundaries. The precipitate of the carbides may partly contribute to the improved mechanical properties.