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.     

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.     

Experimental measurements and modeling of transient heat transfer in forced flow of He II at high velocities

An experiment has been built to study heat transfer in forced flow of He II at flow velocities up to 22 m/s. The main part of this experiment is a 10 mm ID, 0.86 m long straight test section instrumented with a heater, thermometers and pressure transducers. The high flow velocities allow clear observation of the effects of the forced convection, counterflow heat transfer and the Joule–Thomson effect. A numerical model based on the He II energy conservation equation and including pressure effects has been developed to compare with the experimental results. The model works well for low flow velocities where the heat flux is primarily driven by the temperature gradient and for high flow velocities where the heat flux is primarily driven by the pressure gradients. In the intermediate velocity region, discrepancies between the model and experiment may result from an inappropriate representation of the heat flux by counterflow when the temperature and pressure gradients have an effect of similar magnitude on the heat flux.     

Experimental investigation of forced flow boiling of nitrogen in a horizontal corrugated stainless steel tube

Experimental investigation is performed on the heat transfer characteristics of forced flow boiling of saturated liquid nitrogen (LN₂) in a horizontal corrugated stainless steel tube with a 17.6 mm maximum inner diameter. The local heat transfer coefficients (HTCs) are measured at two mass flow rates with a wide range of wall heat fluxes. The effects of the heat flux, mass flow flux and vapor quality on the two-phase heat transfer characteristics are discussed. The results reveal that the local HTCs increase with the heat flux and mass flow flux. The measured local HTCs present a strong dependence on the heat flux. The circumferential averages of the HTCs for the present corrugated tube are compared with the empirical correlations proposed for the smooth tubes, and the results show that the heat transfer is enhanced due to the area augmentation.     

Experimental investigation and correlation of two-phase frictional pressure drop of R410A–oil mixture flow boiling in a 5 mm microfin tube

This study presents experimental two-phase frictional data for R410A-oil mixture flow boiling in an internal spiral grooved microfin tube with outside diameter of 5 mm. Experimental parameters include the evaporation temperature of 5 °C, the mass flux from 200 to 400 kg m^?2 s^?1, the heat flux from 7.46 to 14.92 kW m^?2, the inlet vapor quality from 0.1 to 0.8, and nominal oil concentration from 0 to 5%. The test results show that the frictional pressure drop of R410A initially increases with vapor quality and then decreases, presenting a local maximum in the vapor quality range between 0.7 and 0.8; the frictional pressure drop of R410A–oil mixture increases with the mass flux, the presence of oil enhances two-phase frictional pressure drop, and the effect of oil on frictional pressure drop is more evident at higher vapor qualities where the local oil concentrations are higher. The enhanced factor is always larger than unity and increases with nominal oil concentration at a given vapor quality. The range of the enhanced factor is about 1.0–2.2 at present test conditions. A new correlation to predict the local frictional pressure drop of R410A-oil mixture flow boiling inside the internal spiral grooved microfin tube is developed based on local properties of refrigerant–oil mixture, and the measured local frictional pressure drop is well correlated with the empirical equation proposed by the authors.     

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.     

AC magnetic properties of the soft magnetic composites based on Supermalloy nanocrystalline powder prepared by mechanical alloying

The preparation and characterization of the nanocrystalline soft magnetic composite core based on Supermalloy powder obtained via mechanical alloying route are presented. The AC magnetic properties of the compacts were determined in frequency range from 100 Hz to 100 kHz for flux densities of 0.05 and 0.1 T. Composite materials were obtained by covering the Supermalloy particles with a polymer binder, then compacted into toroidal shape and finally polymerized. It is found that an increase of the compacting pressure from 600 MPa to 800 MPa leads to an increase of the compacts permeability by more than 8%. Also, reducing the polymer content from 2 wt.% to 0.5 wt.% leads to an increase of the magnetic losses (at 100 kHz and 0.1 T) by 380%. The removal of the stresses induced during compaction has been accomplished by a heat treatment at 170 °C for 120 h. This leads to a significant increase (12%) of the relative initial permeability of the compacts.     

Parametric study of unsteady forced convection with pressure gradient

By an extension of differential method, this paper has successfully examined the unsteady forced convection heat transfer from a flow over a flat plate. Transient state is inherent to a sudden change on the heat flux density at the surface of a plate. The general case where the pressure along the direction of flow is not constant is presented. The differential momentum and heat transfer equations are solved numerically. The results are given for different values of pressure gradient parameter m, in the cases of attached boundary layer, and for several values of Prandtl number corresponding to usual fluids (0.71 ⩽ Pr ⩽ 100). The dependences of transient behaviours with Pr number and parameter m are evidenced from the evolutions in time of temperature profile, Stanton number, and duration of unsteady process. Solutions given from the beginning of transient state to the ultimate steady state are discussed. Moreover, analytical solutions, as function of Pr and m, are deduced for Stanton number and duration of unsteady regime.     

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.     

Experimental investigation on the performance of a neon cryogenic oscillating heat pipe

An experimental investigation is conducted to study the performance of a cryogenic oscillating heat pipe (OHP) using neon as the working fluid. The stainless steel OHP with an inner diameter of 0.9 mm has 4 turns, and the lengths of the evaporator, condenser section and adiabatic section are 35 mm, 35 mm and 95 mm, respectively. The temperature of the evaporator and condenser and the pressure of the OHP are measured. The results show that the cooling down process of the OHP from room temperature to the working temperature can be significantly accelerated by charging with neon. During the pseudo steady-state operation process, the temperature of evaporator and the pressure of the OHP increase with increasing heat input. When the dry out appears, the temperature of evaporator rises quickly, and the pressure of the OHP drops sharply. In addition, the effective thermal conductivity of the OHP at the different heat inputs and the different filling ratios is calculated. It increases with increasing heat input, and there exists an optimum filling ratio which makes the maximum effective thermal conductivity. For this OHP, the optimum filling ratio is 24.5%, at which the effective thermal conductivity is 6100–22,180 W/m K.     

Surface treatment of new type aluminum lithium alloy and fatigue crack behaviors of this alloy plate bonded with Ti–6Al–4V alloy strap

Samples consisting of new aluminum lithium alloy (Al–Li alloy) plate developed by the Aluminum Company of America and Ti–6Al–4V alloy (Ti alloy) plate were investigated. Plate of 400 mm × 140 mm × 2 mm with single edge notch was anodized in phosphoric solution and Ti alloy plate of 200 mm × 20 (40) mm × 2 mm was anodized in alkali solution. Patterns of two alloys were studied at original/anodized condition. And then, aluminum alloy and Ti alloy plates were assembled into a sample with FM 94 film adhesive. Fatigue crack behaviors of the sample were investigated under condition of nominal stress σ = 36 MPa and 54 MPa, stress ratio of 0.1. Testing results show that anodization treatment modifies alloys surface topography. Ti alloy bonding to Al–Li alloy plate effectively retards crack growth than that of Al–Li alloy plate. Fatigue life of sample bonded with Ti alloy strap improves about 62.5% than that of non-strap plate.     

Cryogenic flat-panel gas-gap heat switch

A compact additive manufactured flat-panel gas-gap heat switch operating at cryogenic temperature is reported in this paper. A guarded-hot-plate apparatus has been developed to measure the thermal conductance of the heat switch with the heat sink temperature in the range of 100–180 K. The apparatus is cooled by a two-stage GM cooler and the temperature is controlled with a heater and a braided copper wire connection. A thermal guard is mounted on the hot side of the device to confine the heat flow axially through the sample. A gas handling system allows testing the device with different gas pressures in the heat switch. Experiments are performed at various heat sink temperatures, by varying gas pressure in the gas-gap and with helium, hydrogen and nitrogen gas. The measured off-conductance with a heat sink temperature of 115 K and the hot plate at 120 K is 0.134 W/K, the on-conductance with helium and hydrogen gases at the same temperatures is 4.80 W/K and 4.71 W/K, respectively. This results in an on/off conductance ratio of 37 ± 7 and 35 ± 6 for helium and hydrogen respectively. The experimental results matches fairly well with the predicted heat conductance at cryogenic temperatures.     

Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

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.     

Steady state boiling crisis in a helium vertically heated natural circulation loop – Part 1: Critical heat flux,boiling crisis onset and hysteresis

Experiments were conducted on a 2-m high two-phase helium natural circulation loop operating at 4.2 K and 1 atm. The same loop was used in two experiments with different heated section internal diameter (10 and 6 mm). The power applied on the heated section wall was controlled in increasing and decreasing sequences, and temperature along the section, mass flow rate and pressure drop evolutions were recorded. The values of critical heat flux (CHF) were found at different positions of the test section, and the post-CHF regime was studied. The predictions of CHF by existing correlations were good in the downstream portion of the section, however CHF anomalies have been observed near the entrance, in the low quality region. In resonance with this, the re-wetting of the surface has distinct hysteresis behavior in each of the two CHF regions. Furthermore, hydraulics effects of crisis, namely on friction, were studied (Part 2). This research is the starting point to future works addressing transients conducing to boiling crisis in helium natural circulation loops.     

Measurement and correlation of frictional pressure drop of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube

The objective of this paper is to investigate the effect of nanoparticle on the frictional pressure drop characteristics of refrigerant-based nanofluid flow boiling inside a horizontal smooth tube, and to present a correlation for predicting the frictional pressure drop of refrigerant-based nanofluid. R113 refrigerant and CuO nanoparticle were used for preparing refrigerant-based nanofluid. Experimental conditions include mass fluxes from 100 to 200 kg m^?2 s^?1, heat fluxes from 3.08 to 6.16 kW m^?2, inlet vapor qualities from 0.2 to 0.7, and mass fractions of nanoparticles from 0 to 0.5 wt%. The experimental results show that the frictional pressured drop of refrigerant-based nanofluid increases with the increase of the mass fraction of nanoparticles, and the maximum enhancement of frictional pressure drop is 20.8% under above conditions. A frictional pressure drop correlation for refrigerant-based nanofluid is proposed, and the predictions agree with 92% of the experimental data within the deviation of ±15%.     

Joining phenomena and tensile strength of friction welded joint between pure titanium and low carbon steel

This paper describes the effect of the friction welding condition on the joining phenomena and the tensile strength of friction welded joint between pure titanium (P-Ti) and low carbon steel (LCS). The adjacent region of the weld interface at the P-Ti side was intensely upsetting with accompanied large deformation of itself when the joint had sparkle at both applied friction pressures of 30 and 90 MPa, although that of the LCS side was hardly upset. The temperature of the whole weld interface at a friction pressure of 30 MPa reached to 1150 K or over at a friction time of 3.0 s or longer. However, the half radius and centreline portion temperatures of the weld interface at a friction pressure of 90 MPa was not reached to 1150 K, although the periphery portion of that was reached to its temperature. The central portion of the weld interface at a friction pressure of 90 MPa was deformed to a convex shape from the viewpoint of the P-Ti side, although that of 30 MPa remained almost flat after when the friction torque reached the initial peak. When the joint was made at a friction pressure of 30 MPa, a friction time of 3.0 s or longer, and a forge pressure of 270 MPa or higher, it achieved 100% joint efficiency and the P-Ti base metal fracture with no crack at the weld interface. However, many joints at friction times of 1.2 and 1.5 s fractured at the weld interface, although those achieved 100% joint efficiency, because whole weld interface temperature was below 1150 K. On the other hand, many joints at a friction pressure of 90 MPa with high forge pressure also fractured at the weld interface, although those achieved 100% joint efficiency, because the weld interface temperature at the half radius and periphery portions was below 1150 K. Those joints did not have the intermetallic compound layer at the weld interface. The difference of the fractured portion of the joint in both applied friction pressures was due to the difference between the maximum temperature at the weld interface during the friction process and the deformation amount of the LCS side caused by applied forge pressure. To obtain 100% joint efficiency with the P-Ti base metal fracture with no crack at the weld interface, the joint should be made with high forge pressure, low friction pressure, and with opportune friction time at which the temperature at whole weld interface reached around 1150 K.     

Survival of actively cooled microvascular polymer matrix composites under sustained thermomechanical loading

Exposure to high heat can cause polymer matrix composites (PMC) to fail under mechanical loads easily sustained at room temperature. However, heat is removed and temperature reduced in PMCs by active cooling through an internal vascular network. Here we compare structural survival of PMCs under thermomechanical loading with and without active cooling. Microchannels are incorporated into autoclave-cured carbon fiber/epoxy composites using sacrificial fibers. Time-to-failure, material temperature, and heat removal rates are measured during simultaneous heating on one face (5–75 kW/m²) and compressive loading (100–250 MPa). The effects of applied compressive load, heat flux, channel spacing, coolant flow rate, and channel distance from the heated surface are examined. Actively cooled composites containing 0.33% channel volume fraction survive without structural failure for longer than 30 min under 200 MPa compressive loading and 60 kW/m² heat flux. In dramatic comparison, non-cooled composites fail in less than a minute under the same loading conditions.     

Residual stress and strain in MIG butt welds in 5083-H321 aluminium: As-welded and fatigue cycled

This paper presents single-line residual stress profiles for 8 mm 5083-H321 aluminium plates joined by gas metal arc (MIG) welding. The data were obtained by synchrotron diffraction strain scanning. Weld metal stresses (up to ～7 mm either side of the centreline) are quite scattered and unreliable because of the large epitaxial grain size in the fusion zone. Peak magnitude of the transverse stresses varies between +50 MPa (19% of parent plate proof strength) at the HAZ boundary to ?150 MPa (57% of PP proof strength) at the weld centreline. Equivalent values for longitudinal stresses are +90 MPa (34% of PP proof strength) some 22 mm from the weld centreline to ?120 MPa (45% of PP proof strength) at the weld centreline. Plate-to-plate variation in the as-welded transverse and longitudinal residual stress values across the weld toe region is around 40 MPa. The effect on residual stress and strain values of a sequence of applied fatigue loads was also considered and reported.     

Development and parametric study of the convection-type stationary adiabatic demagnetization refrigerator (ADR) for hydrogen re-condensation

The adiabatic demagnetization refrigerator (ADR) system in this paper is composed of a conduction-cooled current cycling high-temperature superconducting (HTS) magnet system, a magnetic bed assembly, its heat exchange parts and an auxiliary precooling stage (a commercial GM cryocooler and a liquid nitrogen vessel). The whole magnetic refrigeration system including the conduction-cooled HTS magnet is cooled by the precooling stage to absorb the rejection heat of the ADR cycle. The packed bed type magnetic bed consists of tiny irregular powders of Dy_0.9Gd_0.1Ni₂ enclosed in a thin walled stainless steel container (22.2 mm in O.D., 0.3 mm in thickness and 40.0 mm in height). The precooled heat transfer fluid (helium) travels through the magnetic material when heat rejection is required; otherwise the helium stagnates within its pores (pseudo-adiabatic process). Flow of the heat transfer fluid substitutes for the function of a traditional heat switch, creating, essentially, a forced-convection type heat switch. The magnetic bed assembly is periodically magnetized and demagnetized at the center of the conduction-cooled HTS magnet which can stably generate both strong and alternating magnetic field from 0 T to 3.0 T (0–130 A) with an average ramp rate of 0.24 T s⁻¹. The cooling capacities of the ADR system at 20 K which is the normal boiling point (NBP) of hydrogen, are 11.1 J cycle⁻¹, 6.3 J cycle⁻¹ and 1.9 J cycle⁻¹ when the temperature spans are 1 K, 2 K and 3 K, respectively. We describe the detailed construction of the ADR system and discuss the test results with the operational parameters (the entrained helium pressure, the mass flow rate of helium and the operating temperature span) in the 20 K region.