Mass Flux Range Research Articles

Development of a nanoscale hot-wire probe for supersonic flow applications

A new nanoscale thermal anemometry probe (NSTAP) was designed and fabricated to measure mass flux in supersonic flows. This sensor was evaluated in the Trisonic Wind Tunnel Munich (TWM) at both subsonic and supersonic speeds. Subsonic compressible flow tests were performed to confirm the new sensor’s repeatability and to compare its behaviour to measurements from a conventional cylindrical hot-wire, while supersonic tests were performed to investigate the nature of the convective heat transfer from the nanoscale sensor at those conditions. For the range of mass fluxes tested in the supersonic regime, a linear relationship between the Nusselt number and the Reynolds number fit the data well. A linear relationship has previously been noticed at length scales close to the molecular mean free path of the flow and has been attributed to the free-molecule flow regime, where the Knudsen number is on the order of unity.

Flow boiling in horizontal annuli outside horizontal smooth, herringbone and three-dimensional enhanced tubes

Heat transfer and pressure drop measurements of R410A saturated flow boiling in horizontal annuli were performed at a saturation temperature of 6 °C over the mass flux range of 8–107 kg/(m2 s). The inner tubes with the same inside diameter of 12.70 mm contain a smooth tube, a herringbone micro-fin tube and a 1EHT tube (a dimpled tube enhanced by petal-shaped background patterns). In this study, the test section is a typical double-tube type heat exchanger and the emphasis is focused on the effects of mass flux, vapor quality, annular width and surface structures on the annulus-side heat transfer. Wilson plot method was adopted to ensure the correction factor of water-side coefficient of 1EHT tube. For flow boiling tests in annuli with an annular gap of 6.35 mm, both the evaporating coefficient and pressure drop increase as the mass velocity increases. Better thermal performance was seen at a outlet vapor quality of 0.6. In this case, the heat transfer rate ratio of enhanced annulus to smooth annulus is 1.7–2.5 for the herringbone micro-fin tube and 1.4–1.7 for the 1EHT tube, respectively. When the annular spacing decreases, heat transfer deterioration will worsen accompanied with a sudden drop in boiling heat-transfer coefficient. Besides, a comparison of flow boiling characteristics of annulus-side and tube-side heat transfer was made. The annulus-side heat transfer coefficient is much larger than that of tube-side case.

Flow and heat transfer characteristics of microchannel flow boiling enhancement with channel configurations

The heat dissipation problem encountered in the process of the development of new electronic equipment and mechanical components towards miniaturization has become the bottleneck of continuous progress in the field of mechanical and electronic control engineering, and the flow boiling heat transfer technology in microchannel may provide the effective cooling guarantee for this purpose. Given the reasonable operating temperature ranges of devices and the diversity of coolant evaporation temperature and pressure choices, refrigerants have become an important option, especially for zeotropic mixtures, which can not only modulate the properties of pure components, but also present the unique phase change heat transfer characteristics with great application prospects. In the present study, the flow boiling heat transfer research of R134a/R245fa zeotropic mixtures as well as their pure compartments in parallel multi-microchannel with continuous and segmented channel configurations were experimentally studied. Each microchannel test section consisted of seven parallel channel passages with the same total length of 110 mm. The cross-sectional area of each channel passage was 2 mm × 1mm (width×height) with the slab width of 0.5 mm. What difference from continuous microchannel was that 10 mm-long interconnected area was arranged every 30 mm along the channel length in segmented one. The experiments were carried out at the same inlet saturation temperature of 26 °C under the conditions comprising the heat flux range of 20− 350 kW/m2 and mass flux range of 300− 400 kg/(m2 s), respectively. In order to obtain more accurate data matching relationship, the analysis of heat transfer performance focused on the region near the outlet of the test sections. It’s worth noting that the multi-microchannel should be treated as fin effects during the heat transfer data reduction. The ribbed slabs could be considered as the rectangular straight fins either for continuous or segmented multi-microchannel. The results showed that compared with continuous microchannel, the heat transfer performance was slightly enhanced for pure components while slightly suppressed for zeotropic mixtures in segmented microchannel at lower effective heat flux. With the increase of effective heat flux, the heat transfer suppression effect of zeotropic mixtures was gradually weakened. The interconnected area was helpful to delay the occurrence of the transfer deterioration and improve the flow boiling CHF regardless of pure or zeotropic refrigerants. Besides, the flow boiling pressure drop in segmented microchannel was obviously reduced for zeotropic mixtures and their pure components. The smaller the liquid-vapor density ratio of test refrigerants, the higher the reduction of pressure drop with the help of interconnected area. For zeotropic mixtures, the flow boiling pressure drop was smaller at lower effective heat flux and the increasing trend of pressure drop on heat flow is slower.

Measurement of heat transfer coefficient and pressure drops in a compact heat exchanger with lance and offset fins for water based Al2O3 nano-fluids

Compact Heat Exchangers (CHEs) are widely used in various applications in thermal fluid-systems including aerospace due to their compactness and light weight coupled with high effectiveness. Among the different types of CHEs for Aircraft Environmental Control System (ECS) and Avionics cooling applications, a cross-flow CHE with Lance & Offset fin is of special interest because of their high heat rejection capability. In the present work, the heat transfer coefficient of water based Al2O3 nano-fluid (NF) flowing in a Compact Plate-fin Heat Exchanger having Lance & Offset fins is studied experimentally. As part of experimental investigation a test setup is designed and developed. Experiments are conducted in the range of mass flux from 50 to 350 kg/s m2 (100 ≤ Re ≤ 600) at room temperature as well as at higher temperatures up to 70 °C. In this study, Al2O3 volume concentration in water is varied from 0.255% to 0.51%. The variation of heat transfer coefficients and pressure drops of Al2O3 nano-fluid with respect to water are presented in graphical form for different mass fluxes. The heat transfer coefficient gets enhanced in low NF temperature regions (between 30 °C-45 °C) up to 25% for 1%NF and up to 35% for 2%NF, where as there is only a marginal improvement at higher operating temperatures (between 60 °C-75 °C). The uncertainty analysis is carried out for heat transfer coefficient based on sensors specification.

Convective boiling of R-410A in 5.0 and 7.0 mm outer diameter microfin tubes

ABSTRACTAlthough microfin tubes are widely used in refrigeration and air-conditioning industries, relatively little data are available for small diameter tubes. In the present study, 7.0 and 5.0 mm outer diameter (O.D.) microfin tubes were tested for a range of mass flux (from 50 to 250 kg/m2s) and vapor quality (from 0.2 to 0.8). The saturation temperature was 8°C and the heat flux was 3.0 kW/m2. This heat flux is quite small, and existing investigations have not been gone to this low heat flux. Results showed that an optimum behavior was observed for the enhancement factor. It increased as mass flux increased up to 150 kg/m2s, and then decreased with a further increase of mass flux. The penalty factors were larger than 1.0 except at the lowest mass flux of 50 kg/m2s. Possible reasoning was provided based on flow patterns of the microfin and the smooth tube. The enhancement factor ranged from 1.24 to 1.93 for the 7.0 mm microfin tube, and 1.63 to 3.26 for the 5.0 mm microfin tube. The penalty factor ranged from 0.82 to 1.11 and from 0.48 to 1.39 for the 7.0 mm and the 5.0 mm microfin tube, respectively. Larger high fin height of the 5.0 mm microfin tube may be responsible for the larger enhancement and penalty factor of the 5.0 mm microfin tube. Comparison with predictions by existing correlations revealed that 7.0 mm microfin tube data were reasonably predicted. However, 5.0 mm microfin tube data were generally underpredicted, probably due to the large fin height of the 5.0 mm microfin tube.

Boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel

This study experimentally investigated the flow boiling heat transfer, pressure drop, and flow pattern in a horizontal square minichannel with a hydraulic diameter of 2.0 mm, and the effects of mass flux, vapor quality, heat flux, and refrigerant properties on the flow boiling characteristics were clarified. The heat transfer coefficient and pressure drop of R32 and R1234yf were measured in a mass flux range of 50–400 kgm−2s−1 at a saturation temperature of 15 °C. The flow pattern of the square minichannel outlet was observed and was classified as plug, wavy, churn, and annular flows. The heat transfer coefficients in the square minichannel were larger than those in the circular minichannel with a similar hydraulic diameter at low mass flux conditions. The heat transfer coefficients of R32 indicated higher values compared with those of R1234yf at same mass flux and qualities. An empirical heat transfer model taking into account the forced convection, nucleate boiling, and thin liquid film evaporation was developed for horizontal square and circular minichannels. The frictional pressure drop of R32 was 1.5–2 times higher than that of R1234yf at same mass flux and vapor quality condition, and the effect of channel shape on the frictional pressure drop was small unlike the boiling heat transfer.

Two-phase heat transfer in horizontal dimpled/protruded surface tubes with petal-shaped background patterns

Experimental investigations were carried out to measure the average heat transfer coefficient and frictional pressure loss of R410A during condensation and evaporation in five newly-developed dimpled tubes and two smooth tubes. These dimpled tubes were called EHT series tubes enhanced by dimple or protrusion arrays and staggered petal-shaped background patterns on their inner surface. Tests were conducted at three condensation temperatures and two evaporation temperatures; over a mass flux range of 60–450 kg/(m2·s); with an inlet vapor quality of 0.8/0.2 and outlet vapor quality of 0.2/0.8. Among the copper test tubes having the same outer diameter, the 1EHTa tube (dimpled surface tube) showed the highest condensing coefficient and pressure drop under the same test conditions. On the contrary, the superior evaporation heat transfer performance was given by the 1EHTb tube (protruded surface tube). In addition, the positive effects of small tube diameter and low saturation temperature on thermal performance have been confirmed. It was also observed that measured heat transfer coefficient and pressure drop of the copper 1EHTa tube were higher in comparison with the stainless steel one. The main reason could be the low thermal conductivity of stainless steel material. A comparison of heat transfer enhancement in these enhanced tubes has been done. Results indicated that both the copper 1EHTa and 1EHTb tubes provided the satisfactory surface thermal efficiency.

Intrinsic hydrogen yield from gasification of biomass with oxy-steam mixtures

In this paper, experimental investigations on oxy-steam gasification of biomass in downdraft packed bed reactor are presented; propagation regimes and the associated hydrogen yield will be the focus of the current work. Steady flame propagation is shown to be established for a range of oxygen mass flux (16–120 g/m2s) covering ‘gasification’ and ‘combustion’ regimes with O2 fractions of 23, 30 and 40% by mass (rest steam). By restricting the upstream bed temperatures between 120 °C (to avoid steam condensation) and 150 °C (to prevent bulk devolatilization of bed), the intrinsic H2 yield from biomass is determined over an equivalence ratio (Φ) range of 3.5 to 1.2. This is shown to correspond to a ‘volatiles’ based equivalence ratio (ϕv) of 2.2 to 1. Interestingly, the H2 yield over this entire range is within 30–40 g/kg of biomass. Using equilibrium calculations, it is shown that the ‘unburnt volatiles’ is the major H2 source when ϕv > 2 and as ϕv → 1, ‘volatiles’ H2 drops close to zero and the major contribution is through the reaction, C + H2O → CO + H2. Increase in char conversion from about 20% at ϕv ∼2.1 to almost 100% as ϕv → 1, the corresponding increase in peak bed temperature and decrease in ‘higher hydrocarbons’ are consistent with the observed H2 yield. The important insight is, operating close to ϕv = 1, under slightly rich condition, leads to tar free exit gas with little or no compromise on H2 yield. This hitherto unknown result is perhaps the reason why all earlier works focused only on highly fuel rich conditions and/or very high steam temperatures (∼800 °C), tolerating higher tar content.

Experimental study of the dryout of a 2D heat-generating model porous medium

Dryout experiments are performed in a quasi two-dimensional heat-generating model porous medium, with bottom flooding. The test section is composed of an array of heating cylinders placed between two ceramic plates. It is held vertically and a liquid (HFE-7000) is injected from bottom at a controlled flow rate, pressure and temperature. Dryout incipient power is investigated by applying an increasing thermal power to a bundle of heating cylinders until a dry zone is detected. The liquid inlet mass fluxes range from 1.0kgm-2s-1 to 10.5kgm-2s-1 and the heat fluxes are between 195kWm-2 and 1086kWm-2. Two kinds of dryout phenomenology are observed. First, at low liquid injection rate (below 4.2kgm-2s-1 inlet mass flux), reaching the dryout power results into a liquid front receding from the top of the test section to the upper limit of the heated zone, while downstream the heated zone, the porous medium is vapour-saturated. Second, at higher flow rate (over 5.2kgm-2s-1 inlet mass flux), the boiling crisis happens at the surface of a single heating element, resulting in a local film boiling, whereas a two-phase flow still go through the whole test section. In both cases, high-speed visualizations allow characterizing 8 boiling/flow regimes, depending on the inlet mass flux, the power released by the heated zone, and the location inside the porous medium. In particular, when reaching the dryout incipient power, the existence of a pulsed flow regime is highlighted. Such a characterization of boiling/flow regimes might be helpful in improving dryout models.

Micro-grooved surfaces to enhance flow boiling in a macro-channel

The influence of the boiling surface morphology on subcooled flow boiling heat transfer is investigated. Flow boiling experiments are conducted in a macro-channel with water entering at 30 °C. The channel has an orthogonal cross-section (10x40 mm) with a short (length: 120 mm) one-sided heated wall. Experiments are performed at two flow directions, horizontal and vertical upward. The examined mass and heat fluxes range between 330–830 kg/m2s and 200–1000 kW/m2, respectively. Two copper boiling surfaces are manufactured by laser etching: one with micro-grooves parallel to the flow direction (surface #1) and one with micro-grooves perpendicular to the flow direction (surface #2). The grooves have the same width (420 μm) and depth (290 μm) but their length varies: 100 mm along the channel’s length (surface #1) and 30 mm across the channel’s width (surface #2). The presence of grooves yields ∼ 8% increase of heat exchange area in both surfaces. A smooth plain copper surface is employed as reference. Micro-grooves lead to boiling inception at lower wall superheats (−70% for horizontal and −30% for vertical channel inclination) and also enhance heat transfer coefficients (10–15% for horizontal and 5–7% for vertical channel inclination) compared to the smooth surface; this is for two reasons: (a) laser etching creates micro-scale-roughness inside the grooves, which provide more active bubble nucleation sites, and (b) the bottom of the grooves is hotter than the rest of the surface. As a result, many bubbles are generated inside the grooves, where they grow and coalescence with other bubbles at a greater extent than the rest of the boiling surface. The beneficial effect of the grooved surfaces is beyond the gain offered by the rise in surface area and it is seen mainly in the horizontal inclination, whereas it is less evident in the vertical inclination. This is comparable with the discrepancy observed between inclinations for the smooth boiling surface.

Experimental investigation of in-tube condensation of low GWP refrigerant R450A using a fiber optic distributed temperature sensor

Heat transfer coefficients and frictional pressure drop values are experimentally measured during condensation of R134a and its proposed low global warming potential (GWP) replacement, R450A. R450A is a non-flammable zeotropic mixture of R134a and R1234ze (42/58% by mass). Experiments were conducted in a horizontal tube (ID = 4.7 mm) for a range of mass fluxes (100 kg m−2 s−1 to 550 kg m−2 s−1) and saturation conditions (45 ∘C and 55 ∘C). The measured pressure drop values and heat transfer coefficients were compared with established literature correlations. Good agreement indicates that existing correlations can be used for design of condensers with R450A as the working fluid. Additionally, this study evaluates the viability of using distributed fiber optic temperature sensors during refrigerant condensation experiments. The fiber sensor is installed in the annulus of a tube-in-tube heat exchanger and is used to obtain the axial temperature profile of the cooling water. The measurements from the distributed temperature sensors are compared with conventional RTD measurements co-located within the test section. The results (Average Deviation = ±0.26 ∘C, Maximum Deviation = ±1.11 ∘C) suggest that fiber optic measurement techniques can provide temperature data with accuracy and high spatial resolution (equal to 0.65 mm).

Experimental Study of Condensation of Steam in Helically Coiled Tubes

Helical coils are most widely used enhancement technique because of its compact structure, low cost and long life. The condensation process of steam inside helical coil is investigated for different mass flux range from 68 to 97 Kg/m2s. and different average vapor quality. Experimental data were plotted against flow maps of Breber and Tandon which matches qualitatively. Heat transfer coefficient and overall HTC is plotted against mass flux and average vapour quality which shows it increases with increase in mass flux and vapor quality of steam.

Experimental investigation on thermal stratification induced by steam-air mixture vertical injection with shallow submergence depth

A series of experiments have been performed to investigate thermal stratification induced by steam-air mixture vertical injection, in the range of steam mass flux 3–30 kg/m2s, air mass fraction 0–5% and submergence depth 100–300 mm. Experimental results show that: thermal stratification occurs at the region where none of chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume is strong enough. For pure steam experiments, thermal stratification occurs when the steam mass flux above 15 kg/m2s. The pool is mixed well firstly, then thermally stratified, and then re-mixed in sequence, as the water temperature increases. A little air can make thermal stratification and re-mixing occur ahead of time and more air can completely remove thermal stratification. Gas bypass induced by shallow submergence depth makes thermal stratification occur earlier and more severe. The submergence depth has little effect on thermal stratification in chugging regime.

Validation of Selected Cesar Friction Models of the ASTECV21 Code Based on Moby Dick Experiments

In the event of a loss of integrity of the main coolant line, a large mass and energy release from the primary circuit to the containment is to be expected. The temporal evolution of such depressurization is mainly governed by the critical flow, whose correct prediction requires, in first place, a correct description of the different friction terms. Within this work, selected friction models of the CESAR module of the Accident Source Term Evaluation Code (ASTEC) V2.1 integral code are validated against data from the Moby Dick test facility. Simulations are launched using two different numerical schemes: on the one hand, the classical five equation (drift flux) approach, with one momentum conservation equation for an average fluid plus one algebraic equation on the drift between the gas and the liquid; on the other hand, the recently implemented six equation approach, where two differential equations are used to obtain the phase velocities. The main findings are listed hereafter: The use of five equations provides an adequate description of the pressure loss as long as the mass fluxes remain below 1.24 kg/cm2 s and the gas mass fractions below 5.93 × 10 − 4. Beyond those conditions, the hypotheses of the drift flux model are exceeded and the use of an additional momentum equation is required. The use of an additional momentum equation leads to a better agreement with the experimental data for a wider range of mass fluxes and gas mass fractions. However, the qualitative prediction for high gas mass fractions still shows some deviations due to the decrease of the regular friction term at the end of the test section.

Experimental and numerical studies on convective heat transfer of supercritical R-134a in a horizontal tube

Experimental studies on convective heat transfer characteristics of supercritical R-134a flowing in a horizontal tube with an inner diameter 8 mm were conducted in the range of mass flux 500–1000 kg m−2 s−1, heat flux 20–100 kW m−2 s−1 and pressure 4.3–4.8 MPa. Local wall temperatures and convective heat transfer coefficients of the top and bottom surfaces of the test tube were obtained, and their variational trends with increasing heat flux and mass flux were discussed within the experimental range. To have a quantitative and more comprehensive analysis of convective heat transfer behaviors of supercritical fluids as compared with the conventional methods, two new field synergy analytical methods designated as TCEH and FCEH respectively were developed, motivated by the two basic concepts of the field synergy principle respectively, i.e., the analogy of convective heat transfer to conductive heat transfer and the three general criteria associated with convective heat transfer enhancements. Numerical simulations were performed within the identical parameter ranges, and the effects of buoyancy, heat flux and mass flux on convective heat transfer were analyzed using these two new methods. Finally some new insights into supercritical convective heat transfer were obtained.

Evaluation of Additively Manufactured Microchannel Heat Sinks

Microchannel heat sinks allow the removal of dense heat loads from high-power electronic devices at modest chip temperature rises. Such heat sinks are produced primarily using conventional subtractive machining techniques or anisotropic chemical etching, which restricts the geometric features that can be produced. Owing to their layer-by-layer and direct-write approaches, additive manufacturing (AM) technologies enable more design-driven construction flexibility and offer improved geometric freedom. Various AM processes and materials are available, but their capability to produce features desirable for microchannel heat sinks has received a limited assessment. Following a survey of commercially mature AM techniques, direct metal laser sintering was used in this paper to produce both straight and manifold microchannel designs with hydraulic diameters of $500~\mu \text{m}$ in an aluminum alloy (AlSi10Mg). Thermal and hydraulic performances were characterized over a range of mass fluxes from 500 to 2000 kg/m2s using water as the working fluid. The straight microchannel design allows these experimental results to be directly compared against widely accepted correlations from the literature. The manifold design demonstrates a more complex geometry that offers a reduced pressure drop. A comparison of the measured and predicted performance confirms that the nominal geometry is reproduced accurately enough to predict pressure drop based on conventional hydrodynamic theory, albeit with roughness-induced early transition to turbulence; however, the material properties are not known with sufficient accuracy to allow for a priori thermal design. New design guidelines are needed to exploit the benefits of AM while avoiding undesired or unanticipated performance impacts.

Subcooled flow boiling of water in a copper microchannel: Experimental investigation and assessment of predictive methods

The current study presents experimental pressure drop and heat transfer data for subcooled flow boiling of water in a 1.0 mm wide × 0.49 mm deep × 40 mm long aged copper microchannel. The experiments were performed for three inlet temperatures: 30o C, 50o C and 70o C, each with mass flux range of 528–1188 kg/m2 s and heat flux range of 260–1100 kW/m2. The trends observed in experimental subcooled boiling heat transfer and pressure drop are reported. The performance of existing predictive methods in predicting pressure drop, heat transfer and boiling incipience is also assessed. The study also evaluates the appropriateness of using singe phase correlations for predicting subcooled boiling pressure drop, a practice which is used in many flow boiling studies, and recommends that the approach be not used for low inlet subcooling.

Performance Augmentation of Single-Phase Heat Transfer in Open-Type Microchannel Heat Sink

Single- and two-phase heat transfers from a microchannel heat sink (MCHS) are emerging as innovative methods of cooling for diversified engineering applications including microelectronics, power production, medical, and chemical industries. However, high-pressure drops and miscellaneous flow boiling instabilities are the major impediments of the technology. Open-type microchannels are recently projected to ameliorate a few of these limitations, including flow boiling instabilities and pressure drop penalties. In the current experimental study, heat transfer performance augmentation of single-phase flow in an open-type microchannel heat sink has been studied. Two configurations of the open-type microchannel heat sink, plain open MCHS and extended open MCHS, were fabricated and tested under a mass flux range of and an effective heat flux range of . Water was used as a cooling medium. It has been observed that fins in the plain open MCHS intensify heat transfer performance by 15%, and a maximum wall temperature reduction of 3.7°C has been observed in the current study for the extended open MCHS. On the contrary, a pressure drop penalty of around 18% has been witnessed for the extended open MCHS due to the additional flow obstacle offered by the extended surfaces. The overall thermal performance of the extended open MCHS indicated that the configurations can become substitutes for the closed MCHS in the future.

High heat flux flow boiling of refrigerant R236fa in parallel microchannels

This paper presents the results of an experimental study of the heat transfer during flow boiling of refrigerant R236fa in a horizontal microchannel heat sink. The experiments were performed using closed loop that re-circulates coolant. Microchannel heat exchanger that contains two microchannels with 2x0.4 mm cross-section was used as the test section. The dependence of average heat flux on wall superheat and critical heat flux were measured in the range of mass fluxes from 600 to 1600 kg/m2s and in the range of heat fluxes from 5 to 120 W/cm2. For heat flux greater than 60 W/cm2, nucleate boiling suppression has significant effect on the flow boiling heat transfer, and this leads to decrease of the heat transfer coefficient with heat flux grows.

Two-phase pressure drop of ammonia in horizontal small diameter tubes: Experiments and correlation

A series of experiments are conducted to investigate the two-phase pressure drop of ammonia in horizontal small diameter tube. The experiments are performed at saturation temperatures of −15.8–4.6 °C, and the mass flux range is from 20 to 100 kg m−2 s−1. This paper analyzes the influence mechanisms of vapor quality, mass flux, saturation temperature, and tube diameter on the two-phase frictional pressure gradient in detail. Meanwhile, several correlations are compared with all the experimental data and the results show that the Müller-Steinhagen and Heck (1986) correlation gives the best agreement. With the analysis of the major forces for ammonia two-phase flow under the tested experimental conditions, the Reynolds number and Bond number are introduced to modify the Müller-Steinhagen and Heck (1986) correlation. Lastly, a modified correlation is developed with the mean absolute deviation of only 13.5%, and 89.2% of the data are within the ± 30% error band. In addition, the modified correlation is evaluated based on a complied database which contains 951 experimental data points from 11 published literatures.