Local pool boiling coefficients on horizontal tubes

Local pool boiling on the outside and inside surfaces of a 51 mm diameter tube in horizontal direction has been studied experimentally in saturated water at atmospheric pressure Much variation in local heat transfer coefficients was observed along the tube periphery On the outside surface the maximum and the minimum are observed at θ=45° and 180°, respectively However, on the inside surface only the minimum was observed at θ=0° Major mechanisms on the outside surface are liquid agitation and bubble coalescence while those on the inside surface are micro layer evaporation and liquid agitation As the heat flux increases liquid agitation gets effective both on outside and inside surfaces The local coefficients measured at θ=90° can be recommended as the representative values of both outside and inside surfaces     

Performance characteristics of a MPCM slurry cooled unit designed for telecommunication equipment

Optimum control of the PCB surface temperature is very important in achieving high performance and operational reliability of telecommunication equipment with high power density and thermal density. In this study, the performance of a liquid cooling unit with MPCM slurries (called as “MPCM cooled unit”) was tested and analyzed. In addition, its performance was compared with that of an air cooled unit and a water cooled unit. The maximum surface temperature and the index of uniform temperature distribution (IUTD) were introduced to analyze cooling performance. The surface temperature in the unit rack of telecommunication equipment can be controlled properly by using an MPCM cooled unit instead of an air cooled unit. The maximum surface temperature and IUTD of the MPCM cooled unit at the inlet temperature of 19°C were lower than those at inlet temperatures of 25°C and 27°C due to the increases of heat capacity and heat transfer rate. The heat capacity of the MPCM cooled unit increased significantly with the increase of mass flow rate due to high specific heat of MPCM particles with latent heat transfer rate. The cooling performance of the MPCM cooled unit was superior to that of the water cooled unit.     

Multiple parameters׳ estimation in horizontal well logging using a conductance-probe array

In this paper, a new method is proposed to simultaneously estimate three parameters in horizontal well logging with oil–water two-phase stratified flow by using a conductance-probe array. It is known that an ideal sensor׳s response is determined by the water level and the azimuth angle of the sensor in a horizontal well, and is proportional to the conductivity of the water phase. In terms of the sensor model, the three parameters are estimated simultaneously by using the conductance probe array by the aid of experimental calibration. Firstly, the sensor is calibrated by static experiments with water of known conductivity. For all possible combinations of water level and azimuth angle, the sensor responses are collected and stored in a sample set as the sample data. In actual use, the sensor is first transported to the desired position in the horizontal well underground and the sensor response is collected as the measured data. Then, through the steps of threshold filtering, data normalization and matching calculation, the sample data that best matches the measured data is searched from the sample set. Finally, the parameters are estimated by the aid of the matched sample. The experimental results show that the estimation errors of water level and azimuth angle are within ±2% and ±3°, respectively and the relative error of water conductivity is no more than ±2%. Moreover, comparing with the estimation method previously proposed by the authors, the new method is of higher reliability, higher accuracy and wider range for water level estimation.     

Freeze-substitution and low-temperature embedding of dairy products for transmission electron microscopy

Dairy products are comprised largely of fat, air and water, which makes it difficult to preserve their ultrastructure for electron microscopy. Keeping the samples frozen throughout fixation and embedding protects the structure and distribution of the components of emulsions and foams. Therefore, dairy products were freeze‐substituted and embedded at low temperature (?20 °C) to prepare them for transmission electron microscopy. Whipped cream, ice cream mix and dairy/non‐dairy mixed systems were frozen by plunging in propane, at its boiling point (?187 °C). Ice cream, because it is already frozen, was fractured into 1‐mm³ pieces in liquid nitrogen and then added to frozen fixative (?196 °C). Fixative solution consisted of glutaraldehyde, osmium tetroxide and uranyl acetate dissolved in either methanol or acetone. When material was to be stained after sectioning the fixative was limited to glutaraldehyde in methanol. The temperature was increased step‐wise from ?80 to ?20 °C. Solvent was replaced with resin; the polar resin Lowicryl HM4, the non‐polar resin Lowicryl HM20, LR White and LR Gold were tested. Samples were embedded and polymerized at ?20 °C using ultraviolet light to cross‐link the resin. Methanol proved to be the most effective solvent for substituting the ice; the hydrophobic resin Lowicryl HM20 was the most effective resin for retaining fat structure following osmium fixation.     

Tribo-oxidation of Self-mated Ti3SiC2 at Elevated Temperatures and Low Speed

The tribological behavior of self-mated Ti₃SiC₂ is investigated from ambient temperature to 800?°C at a sliding speed of 0.01?m/s in air. The results show that at the temperatures lower than 300?°C, friction coefficient and wear rates are as high as 0.95 and 10^?3?mm³/N?m, respectively. With the temperature increasing to 600?°C, both the friction coefficient and wear rates show consecutive decrease. At 700 and 800?°C, friction coefficient and wear rates are 0.5 and 10^?6 mm³/N?m, respectively. According to the wear mechanism, the tribological behavior of Ti₃SiC₂ can be divided into three regimes: mechanical wear-dominated regime from ambient temperature to 300?°C characterized by pullout of grains; mixed wear regime (mechanical wear and oxidation wear) from 400 to 600?°C; and tribo-oxidation-dominated wear regime above 700?°C. The tribo-oxides on the worn surfaces involve oxides of Si and Ti. And, species transformation occurs to these two oxides with the increasing temperature. In the competition oxidation of elements Ti and Si, Si is preferably oxidized because of its high active position in the crystal structure. Additionally, plastic flow is another notable characteristic for the tribological behavior of self-mated Ti₃SiC₂.     

Calibration of hydrogen Coriolis flow meters using nitrogen and air and investigation of the influence of temperature on measurement accuracy

The performance of four Coriolis flow meters designed for use in hydrogen refuelling stations was evaluated with air and nitrogen by three members of the MetroHyVe JRP consortium; NEL, METAS and CESAME EXADEBIT.A wide range of conditions were tested overall, with gas flow rates ranging from (0.05–2) kg/min and pressures ranging from (20–86) bar. The majority of tests were conducted at nominal pressures of either 20 bar or 40 bar, in order to match the density of hydrogen at 350 bar and 20 °C or 700 bar and −40 °C. For the conditions tested, pressure did not have a noticeable influence on meter performance.When the flow meters were operated at ambient temperatures and within the manufacturer's recommended flow rate ranges, errors were generally within ±1%. Errors within ±0.5% were achievable for the medium to high flow rates.The influence of temperature on meter performance was also studied, with testing under both stable and transient conditions and temperatures as low as −40 °C.When the tested flow meters were allowed sufficient time to reach thermal equilibrium with the incoming gas, temperature effects were limited. The magnitude and spread of errors increased, but errors within ±2% were achievable at moderate to high flow rates. Conversely, errors as high as 15% were observed in tests where logging began before temperatures stabilised and there was a large difference in temperature between the flow meter and the incoming gas.One of the flow meters tested with nitrogen was later installed in a hydrogen refuelling station and tested against the METAS Hydrogen Field Test Standard (HFTS). Under these conditions, errors ranged from 0.47% to 0.91%. Testing with nitrogen at the same flow rates yielded errors of −0.61% to −0.82%.     

Sliding Wear Characteristics of AZ91D Alloy at Ambient Temperatures of 25–200 °C

The dry sliding wear tests were performed for AZ91D alloy under the loads of 12.5–300 N and the ambient temperatures of 25–200 °C. We studied the wear characteristics of AZ91D alloy as a function of the normal load and the ambient temperature. The mild-to-severe wear transition occurred with increasing the load and the critical load reduced with the ambient temperature rising. However, no matter how high the ambient temperature was in the range of 25–200 °C, the mild wear prevailed under the lower loads. Especially, the AZ91D alloy presented a lower wear rate at 200 °C than at 25 and 100 °C under the low loads of 12.5–25 N, but vice versa under the loads of more than 25 N. These phenomena seem to be contradictory to the popular view that the mild-to-severe wear transition is controlled by the critical surface temperature. These may be attributed to a thick and hard mechanical mixing layer (MML) containing the mixture of MgAl₂O₄ and Mg on the worn surface. The MML thickened with increasing the ambient temperature (under the low loads), effectively reduced wear and markedly elevated the critical surface temperature. The oxidative wear and delamination wear successively predominated in the mild wear regime; the gross plastic-induced wear would prevail in the severe wear regime.     

Effect of temperature on microstructural evolution and subsequent enhancement of mechanical properties in a backward extruded magnesium alloy

The capability of backward extrusion (BE) method was assessed to achieve modified structures in AZ80 magnesium alloy. At first, 3D-Deform was employed to simulate the deformation flow through the deformed cup which gives an evidence from the flow behavior of the material. The material was processed via BE method at various temperatures of 250, 350, and 450 °C. Metallographic investigations were conducted in three different regions of the BE-processed cup (wall, bottom, and flow channel). The main feature observed at the wall of the BE cup was the presence of mechanical twins, the frequency of which was reduced by raising the process temperature. The flow localization in the form of shear banding occurred within the flow channel at all deformation temperatures. The bottom of the BE-processed cup at 250 °C exhibited coarse initial grains along with a continuous network of the eutectic phase at grain boundaries. However, increasing the process temperature to 350 and 450 °C led to the fragmentation of the γ-Mg₁₇Al₁₂ network to fine particles, where a considerable grain refinement was also traced, particularly at 450 °C. Furthermore, a special testing technique, called the shear punch testing method, was utilized to examine the room temperature mechanical properties of the BE specimens. Results indicated that BE-processed materials would benefit from a higher strength in comparison to the initial material; conversely, the ductility follows a different trend depending on the deformation temperature.     

Quantitative analysis of a two-dimensional ice accretion on airfoils

This paper presents the development of a code that can determine the shape of accreted ice on a 2D airfoil, verification of the code via quantitative parameters, and the variation in ice accretion according to ambient conditions. First, the 2D panel method is used as the aerodynamic solver, and Messinger’s model is used as the thermodynamic model. Second, the code is quantitatively verified through comparison with existing ice accretion analysis codes under rime, mixture, and glaze ice conditions. The parameters for comparison are the cross-sectional area of the ice, maximum ice thickness, ice heading, and distribution of the ice thickness measured on the airfoil surface. The verification shows that the developed code yields results of similar accuracy to existing analysis codes. Finally, ice shapes, depending on variations in the ambient conditions, are determined and investigated based on these parameters for comparison. The selected ambient condition parameters are freestream velocity, LWC, and temperature. The investigation is carried out for rime and glaze conditions. Increasing the freestream velocity produces an ice horn that increases the area over which the ice encounters liquid water in the air. The ambient temperature is the factor that alters ice accretion behavior; increasing the ambient temperature turns rime ice into glaze ice. Ice accretion area is increased at higher LWC. The LWC and the ice cross-sectional area show a linear relationship.     

Experimental study of frost growth on a horizontal cold surface under forced convection

Frost formation on a horizontal copper surface under low air temperature and forced convection conditions is investigated experimentally. Both the frost crystals pattern and the frost layer thickness formed on the cold plate are compared under different experimental conditions. The environmental variables considered in this study include the ambient temperature (T _∞ ), air relative humidity (φ), and velocity (v), as well as the cold surface temperature (Tw). The tested ranges are −5≤T _∞ ≤5 °C, 50%≤ φ≤80%, 2.2≤v≤8.0 m/s, −16.8≤T _w ≤−25.5 °C. The experimental results show the cold surface temperature and the air relative humidity have obvious effects on the frost growth: the frost layer thickness increases strongly with the decreasing cold surface temperature and increasing air relative humidity. The air temperature and air velocity or Reynolds number are also important factors affecting the frost crystals’ growth and thickness. With the increase of the air temperature and velocity, the frost crystals become denser, and the frost layer thickness become thicker, but this trend becomes weaker under higher air temperature and velocity.     

Evaluation of thermal debinding of injection-molded boron carbide in an ambient atmosphere

Characterization of thermal debinding of boron carbide green samples in an ambient atmosphere with the temperature range of 350 to 500 °C was investigated. Binders are easier to remove in the ambient atmosphere because of the continuous oxygen supply. XRD results show that the diffraction peak of B₂O₃ appears in debound samples above 350 °C. The diameter and mass of the samples display no significant changes when the debinding temperature is below 400 °C. Moreover, glassy phases are not observed through the analysis on fracture surfaces by scanning electron microscope and debound samples have enough strength for the handling. The linear shrinkage and mass growth of debound samples improve markedly with increasing debinding temperature. Glassy phases are observed when debinding temperature is above 500 °C. Furthermore, the linear shrinkages of these sintered samples debound in the ambient atmosphere below 400 °C are about 18 % which are consistent with the sintered samples debound beforehand in Ar. Therefore, thermal debinding in an ambient atmosphere under suitable temperature (400 °C) is feasible for powder injection-molded B₄C material.     

Freeze-drying of articular cartilage: Investigation of rat femoral heads by SEM

The results of comparative investigations of freeze-drying of joint cartilage which had not been separated from the underlying bone are reported. Fixation and ethanol and amyl acetate substitution procedures cause marked loss of ground substance and thus create surface structures which are not present to the same degree in cartilage which is simply freeze-dried. Indeed, it is questionable whether these structures are present in vivo. They were most highly developed in critical-point-dried cartilage. The main difficulty with freeze-drying is the prevention of damage caused by ice crystals. A tissue temperature of ?130°C should always be aimed for and, because of the rapid growth of ice crystals, the temperature should never be allowed to rise above ?90°C. These conditions can be fulfilled if the specimen is in good thermal contact with the cold stage and if the cooling device is equipped with a condenser which is cooled with liquid nitrogen. The better the tissue was processed, the smoother was the resulting cartilage surface and the greater was the degree to which the chondrocytes fitted the walls of their lacunae. At tissue temperatures above ?90°C, the first change which became apparent was destruction of the cell membranes. This was followed by almost complete elution of the ground substance from between the fibers and, finally, the cell nuclei were also destroyed. This damage caused by ice crystals exposed fibrous structures on the surface of the cartilage, and the appearance of these structures was superior to that produced by enzymatic methods of preparation. The disruption caused by freezing uncovered intracellular structures.     

Tribological Evaluation of an Al2O3-SiO2 Ceramic Fiber Candidate for High Temperature Sliding Seals

A test program to determine the relative slitting durability of an alumina-silica candidate ceramic fiber for high temperature sliding seal applications is described. Pin-on-disk tests were used to evaluate the potential seal material by sliding a tow or bundle of the candidate ceramic fiber against a superalloy test disk. Friction was measured during the tests and fiber wear, indicated h the extent of fibers broken in the tow or bundle, was measured at the end of each test. Test variables studied included ambient temperature from 25° to 900°C, loads from 1.3 to 21.2 N, and sliding velocities from 0.025 to 0.25 m/sec. In addition, the effects of fiber diameter and elastic modulus on friction and wear were measured. Thin gold films deposited on the superalloy disk surface were evaluated in an effort to reduce friction and wear of the fibers.

In most cases, wear increased with test temperature. Friction ranged from 0.36 at 500°C and low velocity (0.025 miser) to over 1.1 at 900°C and high velocity (0.25 m/sec). The gold films resulted in satisfactory lubrication of the fibers at 25°C. At elevated temperatures diffusion of substrate elements degraded the films. These results indicate that the alumina-silica (Al₂O₃SiO₂) fiber is a good candidate material system for high temperature sliding seal applications. More work is needed to reduce friction.     

Thermal characteristics of a multichip module using PF-5060 and water

The experiments were performed by using PF-5060 and water to investigate the thermal characteristics from an in-line 6 x 1 array of discrete heat sources for simulating the multichip module which were flush mounted on the top wall of a horizontal, rectangular channel of aspect ratio 0.2. The inlet temperature was 15°C for all experiments, and the parameters were the heat flux of simulated VLSI chips with 10, 20, 30, and 40W/cm² and the Reynolds numbers ranging from 3,000 to 20,000. The measured friction factors for PF-5060 and water gave a good agreement with the values predicted by the modified Blasius equation within ±6%. The chip surface temperatures for water were lower by 14.4-21.5°C than those for PF-5060 at the heat flux of 30W/cm². From the boiling curve of PF-5060, the temperature overshoot at the first heater was 3.5°C and was 2.6°C at the sixth heater. The local heat transfer coefficients for water were larger by 5.5-11.2% than those for PF-5060 at the heat flux of 30W/cm², and the local heat transfer coefficients for PF-5060 and water reached a uniform value after the fourth row. This meant that the thermally fully developed condition was reached after the fourth row. The local Nusselt number data gave the best agreement with the values predicted by the Malina and Sparrow’s correlation and the empirical correlations for Nusselt number were provided at the first, fourth and sixth rows for a channel Reynolds number over 3,000.     

Thermal Management Techniques for Oil-Free Turbomachinery Systems

Tests were performed to evaluate three different methods of utilizing air to provide thermal management control for compliant journal foil air bearings. The effectiveness of the methods was based on bearing bulk temperature and axial thermal gradient reductions during air delivery. The first method utilized direct impingement of air on the inner surface of a hollow test journal during operation. The second, less indirect method achieved heat removal by blowing air inside the test journal parallel to the shaft axis to simulate air flowing axially through a hollow shaft. The third method emulated the most common approach to removing heat by forcing air axially through the bearing's support structure. Internal bearing temperatures were measured with three type K thermocouples embedded in the bearing that measured general internal temperatures and axial thermal gradients. Testing was performed in a 1 atm, 260°C ambient environment with the bearing operating at 60 krpm, and supporting a load of 222 N. Air volumetric flows of 0.06, 0.11, and 0.17 m³/min at approximately 150 to 200°C were used.

The tests indicate that all three methods provide thermal management but at different levels of effectiveness. Axial cooling of the bearing support structure had a greater effect on the bulk temperature for each air flow and demonstrated that the thermal gradients could be influenced by the directionality of the air flow. Direct air impingement on the journal's inside surface provided uniform reductions in both bulk temperature and thermal gradients. Similar to the direct method, indirect journal cooling had a uniform cooling effect on both bulk temperatures and thermal gradients but was the least effective of the three methods.     

The effect of ambient air condition on heat transfer of hot steel plate cooled by an impinging water jet

It has been observed that the cooling capacity of an impinging water jet is affected by the seasonal conditions in large-scale steel manufacturing processes. To confirm this phenomenon, cooling experiments utilizing a hot steel plate cooled by a laminar jet were conducted for two initial ambient air temperatures (10°C and 40°C) in a closed chamber, performing an inverse heat conduction method for quantitative comparison. This study reveals that the cooling capacity at an air temperature of 10°C is lower than the heat extracted at 40°C. The amount of total extracted heat at 10°C is 15% less than at 40°C. These results indicate the quantity of water vapor, absorbed until saturation, affects the mechanism of boiling heat transfer.     

Effect of Operating Temperature on Tribological Behavior of As-Plated Ni-B Coating Deposited by Electroless Method

The present work investigates the tribological behavior of electroless Ni-B coating in its as-plated condition at elevated operating temperatures. Ni-B coating is deposited using an electroless method on AISI 1040 steel specimens. Coating characterization is done using scanning electron microscopy, energy-dispersive X-ray analysis, and X-ray diffraction techniques. Vicker's microhardness and surface roughness are measured. Friction and wear tests are carried out on a pin-on-disc tribological test setup at room and elevated temperatures of 100, 300, and 500°C. The tribological behavior deteriorates at 100°C compared to room temperature. Electroless Ni-B coating shows excellent wear resistance at 300°C, which again degrades at 500°C due to severe oxidation and softening of the deposits. The worn surface of the coatings is analyzed using optical microscopy and scanning electron microscopy. Within the temperature range considered, the wear mechanism changes from adhesion to a combination of adhesion and abrasion as the temperature rises from ambient condition to 100°C, following which the wear mechanism is predominantly abrasive. The formation of a tribochemical oxide film also affects the tribological behavior of the coatings at high temperature.     

Calibration of an ultrasonic flow meter for hot water

If we want to keep the number of necessary characterisation measurements within acceptable limits, we need to be confident that a flow instrument design reacts in a predictable and straightforward way to systematic influences. In this paper, the important systematic influences for an ultrasonic flow meter (UFM) for feed water flow are identified to decide which characterisations have to be carried out in addition to a typical baseline calibration with water at 20 °C. In heat metering applications where there are temperatures up to 120 °C it is for example known that the temperature influence on the flow instrument is important and this also applies to higher temperatures such as in the feed water control of power plants. One of the critical systematic temperature influences that affects most flow instruments is the thermal expansion of the meter body. From June 2009 to March 2010, the “Heat and Vacuum” department of the Physikalisch-Technische Bundesanstalt conducted a measurement campaign to characterise the influence of thermal expansion of a meter body on the calibration of an 8 inch (DN 200) five chord UFM for feed water application in the temperature range from 4 °C to 85 °C and flow range from 50 m³ h⁻¹ to 900 m³ h⁻¹. An overview of the procedures and facility used for the calibration is given and the measurement conditions under which the calibrations were performed are detailed. It is shown that a linear model of the thermal expansion effect is appropriate for the investigated conditions.     

Surface Films and Soluble Degradation Products Formed in a PFPAE Fluid and Their Implications to the Wear of Steel

The soluble degradation products were generated in a linear perfluoropolyalkylether (PFPAE) fluid in boundary lubrication. Perfluoropolyalkylether carboxylic acid species were found in the residual fluids from the sliding tests by vibration spectroscopy. Surface-bound organic and inorganic reaction products were identified by vibration microspectroscopy with a grazing angle objective attachment and X-ray photoelectron spectroscopy (XPS), respectively. Inorganic surface films were found to be composed primarily of FeF₃. A monodentate perfluorocarboxylate surface species was found on the sliding surfaces in 50° and 100°C tests but not found in 150°C tests. The higher friction and wear in 150°C tests as compared to 50° and 100°C tests were attributed to the absence of the perfluorocarboxylate species over the sliding surfaces at high temperatures.     

Horizontal bottomed semi-cubic parabolic channel and best hydraulic section

This paper presents a new channel section having semi-cubic parabolic sides and horizontal bottom. The formulae for calculating the area, wetted perimeter are presented. The best hydraulic section is derived using three variables (water depth, water surface width and horizontal bottom width). Results show the ratios of the water surface width to depth, bottom width to depth and water surface width to bottom are all constant for the best hydraulic section. Explicit equations of the best hydraulic section for design are also deduced. Examples show these explicit equations are convenient for design. This type of best hydraulic section is compared with the trapezoid and classic semi-cubic parabolic sections. Results indicate that the area and wetted perimeter are less than those of trapezoid and classic semi-cubic parabolic sections for a given flow discharge. It means less lining and excavation cost is required for construction.