Supplied Heat Flux Research Articles

Heat transfer in inhomogeneous dispersed systems based on graphene oxide hydrogels

Based on the optical holography method, studies of the occurrence and development of convective flows in hydrogels of various concentrations with the addition of graphene oxide in relation to 3D-bioprinting technology have been performed. For quantitative measurement of temperature fields, the optical holography method was used in combination with the gradient thermometry method, based on the dependence of the refractive index on the properties of hydrogel systems modified with graphene oxide with different concentrations and temperatures. Under conditions of changes in the thermophysical properties of hydrogels, as well as the magnitude of the supplied heat flux, the features of heating the wall area are studied in order to determine the coefficients of thermal conductivity and heat capacity, as well as the nature of the formation of convective flows near the wall heated from below.

Advanced Phase-Change Intermediate Heat Exchanger Development for Multistage Thermoelectric Heat Pumps

The need to reach a full energy decarbonisation is well known. Heating and cooling consumption is almost half of the global energy end-use. Thus, development of low-carbon and highly efficient power-to-heat technologies must be developed. In this work, the use of thermoelectric technology working as a heat pump is proposed to heat up an airflow of 38 m3/h. Two different prototypes of multistage thermoelectric heat pumps have been developed and compared based on monophasic and phase-change intermediate heat exchangers. The reduced thermal resistance obtained for the novel phase-change heat exchanger increases the heat flux supplied to the airflow and reduces the consumed power of the system, outperforming the operation of the monophasic thermoelectric heat pump between a 30 and a 67 %. The novel multistage phase-change heat pump obtains experimental COP values between 3.25 and 1.26 when the airflow rises its temperature from 3.5 °C to 23.5 °C. Additionally, this experimental study proves a new methodology to calculate the supplied heat flux to the airflow. The validation of this technology proves a discrepancy of ± 9 % when this novel technology is compared to the conventional one based on the airflow temperature rise.

Coupled bubble dynamics and interaction mechanisms of adjacently nucleated vapor bubbles under subcooled pool boiling regime

The present work investigates the plausible modes of interaction between two adjacently located vapor bubbles growing on high wettability surface during subcooled nucleate pool boiling. The study becomes important as the phenomenon of bubble interaction has a significant impact on the resulting bubble dynamic parameters as well as on microlayer dynamics of the respective vapor bubbles. As against the symmetric growth of microlayer in the case of isolated pool boiling, microlayers of mutually interacting vapor bubbles no longer remain symmetric and tend to deplete relatively faster, thereby influencing the resulting wall heat transfer rates. Toward this, the microlayers and dynamics of adjacently located interacting vapor bubbles have been mapped in a simultaneous manner using thin-film interferometry and high-speed rainbow schlieren deflectometry as a function of varying subcooling levels. Results of the experiments revealed that parameters such as non-dimensional spacing between the two vapor bubbles, supplied heat flux, and degree of subcooling have a strong impact on the mechanism of bubbles' interaction, which, in turn, influence the bubble departure frequency, bubble diameter, and microlayer dynamics. Three dominant modes of bubble interaction, hydrodynamic interaction (HI), thermal interaction (TI), and coalescence (C), were identified as a function of the level of subcooling. Quantification of the experimental data showed that the equivalent diameter of the vapor bubble increases with an increase in the level of heat flux, while it decreases with an increase in the subcooling level. Additionally, the secondary nucleation affects the liftoff time and movement of the primary bubble and can hydrodynamically deplete the microlayer of the first bubble even when the bubbles are far apart, with the depletion rates showing a strong dependence on the growth rate and location of the secondary nucleation.

Thermo-fluidic behaviour of diamond and hourglass microchannels: Investigation from the second law viewpoint

The potential of diamond and hourglass microchannels from the second law perspective is explored to amplify the effectiveness of the thermal device for various thermodynamic systems. The chosen design variables cover all relevant physical traits, such as the divergence-convergence effect (divergence-convergence angle, width ratio), geometrical considerations (depth, length), input variables (Reynolds number, supplied heat flux), and conjugate effect (substrate material and thickness). The performance is quantified in terms of thermal and frictional entropy generation, total entropy generation, and augmented entropy generation number. Results reveal that the proposed geometries outperform uniform microchannels for all cases except at higher Reynolds numbers for high divergence-convergence angle, width ratio, length, and low supplied heat flux. The outcome also demonstrates that the diamond microchannel performs slightly better than the hourglass microchannel for all cases except at shorter lengths and higher aspect ratio for Reynolds numbers above 100, owing to flow recirculation zones and the conjugate effect. Additionally, the first and second law attributes of diamond/hourglass microchannels are compared and analyzed. Diamond/hourglass microchannels with superior second law performance at a given design constraint need not exhibit better first law performance simultaneously, and vice versa. Lastly, the current investigation sheds light on design possibilities that enhance the potential of proposed geometries through prudent exploitation of the physical characteristics. The outcomes of this research have the potential to design diamond/hourglass microchannels with the appropriate geometry for a plethora of heat transfer applications.

Non-intrusive diagnostics of differentially-heated two-component immiscible fluid layer with phase change

Experiments on buoyancy-driven convective phenomena in a differentially-heated two component immiscible fluid layer with phase change phenomena have been reported. The real-time mapping of the transients associated with the process has been affected in a complete non-intrusive manner using rainbow schlieren deflectometry. Experiments have been performed for varying heat flux levels supplied to the lower horizontal surface of the cavity and for different heights of the secondary fluid layer. The two-component immiscible fluid mixture comprises of water as the (lighter) primary fluid layer and dichloromethane (DCM) as the (heavier) secondary fluid component. The supplied heat flux levels are sufficient enough to induce phase change in the secondary fluid layer through nucleate pool boiling. In addition to the real-time mapping of the buoyancy-induced convection in both the fluid layers, the process of vapor bubble inception, their growth and departure from the lower heated wall of the cavity have been recorded in the form of time-lapsed schlieren images. Conditions under which the continued interaction of the vapor phase of DCM with the interface leads to interface deformation and its final breakage have been investigated. Based on the force balance analysis, the critical volume of the DCM vapor phase required for interface breakage has been determined. Dependence of parameters such as neck breakage height, time required for interface breakage etc. on the supplied heat flux level and height of the secondary fluid has been investigated. The experimental observations revealed the importance of vapor bubbles-induced fluid agitation in the otherwise stagnant primary fluid layer, disruption of the thermal boundary layer in a completely passive manner as the upward rising vapor volume condenses at the top horizontal surface of the differentially-heated cavity and their implications in possible heat transfer enhancement.

Flow Instability of a Liquid Through a Small-Scale Channel During a Flow Evaporation

Finite volume analysis of two-dimensional model has been performed to study of the influence of heat flux and mass flux on two phase heat transfer through microchannel during water flow boiling. The channel dimensions were (5×5××50) m3. The inlet boundary conditions were (1.63, 3.32, 4.9, 6.65, 8.16) . The supplied heat fluxes were 24.8 , 33.2 , 46.13 , 51 and 69 for nucleate boiling while 33.3 , 50 , 66.7 , 83.3 , and 100 for dryout condition. The inlet temperature was 30oC. It was shown that the highest heat transfer coefficient occurs at beginning of onset of nucleate boiling (ONB) after that the it will drop due the changing of the flow patterns. According to supplied heat flux, the patterns were bubble flow, dispersed bubbles, churn flow, annular flow, and then dry out condition occurs. For the mass flux 1.63kg/ the highest heat transfer coefficient was 9.5 kW/ while for the mass flux 8.16 kg/ the highest heat transfer coefficient was 20kW/ . For the mass flux 1.63kg/ the highest value of vapour quality was 0.09 while for the mass flux 8.16 kg/ the highest value was 0.15. There is a sharp pressure drop when starts to nucleate and the highest value occurs at dryout condition. This study gave an understanding of the heat exchange flow behaviour during water boiling through a single microchannel.

Intensification of heat transfer in the zone of the thermosyphon evaporator when changing the boiling surface shape

In present paper the influence of the boiling surface treatment types of thermosyphon evaporator on the heat transfer coefficient was studied. An experimental setup «Loop thermosyphon with replacing lower part of the evaporator» has been developed and assembled. This feature makes it easy to replace samples with various surface modifications as the lower part of the evaporator. The description of the experimental setup, equipment and research methods is given. Heat transfer coefficients for the samples at various applied thermal loads (from 5 to 200 W) were calculated. In case of flat aluminum plate (boiling surface) concentric grooves and uneven coating of aluminum oxide particles, an increase of the heat transfer coefficient from h1= 5760 W/(m2·K) to h2= 28339 W/(m2·K) at the supplied heat flux density q = 250 kW/m2was observed. The heat transfer coefficient for a sample without concentric grooves, but with an uneven coating of aluminum oxide particles was h3= 16952 W/(m2·K) at q = 250 kW/m2. Results of the study can be used for further increase of thermosyphon evaporator efficiency.

Generalized energy model of the open thermodynamic system “vehicle fuel tank”. Processes of non-stationary heat transfer with a variable fuel mass

Introduction (statement of the problem and relevance). While gasoline-powered vehicles generate a significant amount of hydrocarbons in the form of the fuel system emitted vapors, the main element of the system being the fuel tank, modern requirements for evaporative emission limits are significantly tightened. At the same time, the vaporization process parameters and the fuel vapor amount are determined by the dynamics of fuel heating in the tank under various modes of vehicle operation.The purpose of the research was to develop a “vehicle fuel tank” energy model, seeking to create an open thermodynamic system which can exchange matter and energy with the environment depending on the variable amount of fuel in the tank.Methodology and research methods. The analysis of heat flush connected to the fuel tank and taken away from it was being carried out. As a result of solving equations for open and closed thermodynamic systems, the parameters characterizing the thermal properties of the fuel tank were obtained.Scientific novelty and results. Additional complex parameters have been proposed, the main of which are: heat transfer of the tank; tank heat capacity; supplied heat flux; the rate of heat capacity change; tank emptying time; fuel heating acceleration factor; the maximum rate of change in temperature difference. To assess the heat and power properties of the fuel tank, an additional parameter of the sphere surface area ratio to the surface area of the same volume tank has been proposed, which allowed estimating the fuel tank heat transfer to the environment.Practical significance. Equations have been obtained that allow estimating the level of fuel temperature depending on the thermal properties and shape of the fuel tank in the absence and presence of fuel pump control.

ALGORITHMS AND SOFTWARE OF BUILDINGS INTERMITTENT THERMAL CONDITIONS OPTIMAL CONTROL

The actual problem of synthesizing the optimal control law for the intermittent heating mode of office buildings is considered, which consists in the programmed reduction of the room temperature during off-hours. The analysis of information sources showed the absence of sufficiently justified methods for solving the problem of optimizing the thermal processes of buildings, which provide comfortable conditions for the functioning of personnel, on the one hand, and save costs for buildings maintenance, on the other hand. The statement of the building thermal state optimal control problem is formulated, which contains a mathematical model of the process, a control goal, constraints on control actions and an integral quadratic quality criterion, including deviations of the room temperature from a given program and heat consumption. The limitation on the control action is taken into account by the method of penalty functions. As a mathematical model of the optimized thermal process, a two-dimensional controlled system of differential equations with constant coefficients is applied. The values of these coefficients are determined on the basis of building similarity criteria, which allows solving the optimization problem in a dimensionless form. The model includes a control action - the supplied heat flux, and a disturbing action - the ambient temperature, changing according to a given harmonious law. The numerical solution of the optimal control problem is implemented by the Chernous'ko method of successive approximations. A description of the user interface is given, as well as examples of the implementation of the thermal state optimal control. The proposed software allows on the stage of designing the heating system to obtain express assessments of various thermal modes of the building, and can also be used as computer support for automated individual heating points.
 Keywords: intermittent heating mode, optimal control, mathematical model, control quality criterion, control constraints, penalty function, successive approximations, program interface, automated individual heating point.

Experimental study on the advective heat flux of a heat exchanger for passive cooling of spent fuel pools by temperature anemometry grid sensor

In commercial nuclear power plants spent fuel assemblies are usually stored in actively cooled water pools. The continuous decay heat release represents a potential risk in case of a station black out scenario. Thus two-phase passive heat removal systems are a key technology to enhance the safety of nuclear power plants. Such systems work only by the energy provided from the heat source, e.g. by the maintenance of a natural convection cooling. A heat transfer loop using air as an unlimited heat sink consists of a primary heat exchanger in the spent fuel pool water and a secondary heat exchanger located in ambient air. Thus the measurement of the heat flux, which gets transferred from the pool to the ambient air, is an important task. If one would measure heat flux, flow rates and temperatures in many positions by help of local probes, the natural flow would get strongly disturbed. For that reason we introduce a heat flux measurement around the secondary heat exchanger located in ambient air, which applies temperature and velocity measurement by an anemometric principle.A 6.5m long flow channel with an electrical heated finned tube heat exchanger was set up at the TOPFLOW facility at HZDR. Since the tubes of a heat exchanger would be tilted in a passive heat removal system, i.e. to allow drainage of the condensed heat transfer medium, different tiled angles were adjusted to 0° (horizontal), 20°,30° and 40°. The frontal velocity was varied between 0.5ms and 4ms and three thermocouples were placed up- and downstream of the heat exchanger respectively. A Temperature Anemometry Grind Sensor (TAGS) was located downstream the heat exchanger. It consists of a wire grid with platinum resistance elements, which are placed in the small sub-channels of a flow straightener to generate laminar flow profiles. Two methods were used to calculate the heat flux: arithmetical average and weighting of the flow area. The results of velocity was compared with the average velocity measured by the volume flow control and out of the velocity and temperature the heat flux was calculated and compared with electrical supplied heat flux. The calculated average velocity measured by the TAGS corresponds well with the velocity measured by the volume flow controller up to approximately 3ms with a maximum deviation of ±5%, but underestimates the velocity measured by the volume flow controller at higher velocities. The heat flux was calculated by five methods, 1.) from the three thermocouples up- and downstream of the heat exchanger, 2.) from the average temperatures measured by the TAGS, 3.) from the weighted temperature measured by the TAGS, 4.) from the average temperature and velocity measured by the TAGS and 5.) from the weighted temperature and velocity measured by the TAGS. In this order the accuracy of methods increases compared to the electrical supplied heat flux. For the last method the maximum deviation was 6.5% for all tilt angles. This measurement concept determines the heat flux without disturbing the flow in the loop.

Nucleate boiling heat transfer of R-134a and R-134a/POE lubricant mixtures on smooth tube

The present study examines the influence of polyolester lubricant oil (POEA-68 and POEA-170) with R-134a on the nucleate pool boiling performance for a horizontal smooth tube. The heat flux ranges from 10 kW/m2 to 90 kW/m2 and the lubricant mass fraction varied from 1% to 10% at a saturation temperature of 10 °C, 0 °C and, −6 °C, respectively. The experimental results show that the heat transfer coefficient (HTC) of the pure refrigerant is increased with an increase in saturation temperature. But the presence of lubricant oil alters the HTC considerably. The addition of 3% POEA-68 lubricant shows the highest 29.3% HTC enhancement relative to pure refrigerant at a supplied heat flux of 70 kW/m2 and at a saturation temperature of 0 °C. Whereas for R-134a/POEA-170 lubricant mixture at 3% of mass fraction, the highest enhancement in HTC was around 26% at a supplied heat flux of 70 kW/m2 and −6 °C saturation temperature. The bubble size is reduced and the bubble density is increased appreciably with the addition of POE oil. It was observed that the HTC of lubricant-refrigerant mixtures differs appreciably with the variation of lubricant oi mass fraction (ω). The HTC first decreases at a very low mass fraction (ω≤1%), followed by an appreciable increase in the range of 1% ≤ ω ≤ 3%, and finally reveals a continuous decline for the higher mass fraction. The dramatic decrease in HTC for lubricant-refrigerant mixture is associated with mass transfer resistance and a significant increase in surface tension.

Investigation of boiling heat transfer of titania nanofluid flowing through horizontal tube and optimization of results utilizing the desirability function approach

A flow boiling experimental investigation was performed to examine the heat transfer coefficient of water and water-TiO2 nanofluid. During investigation, the Reynolds number varied from 8000 to 24,000, pressure from 1 to 2.5 bar, and volumetric concentration of nanofluid from 0.05 to 0.20%. Supplied heat flux was 143 kWm−2. Experimental results of water were in good agreement with CFD investigation and Chen correlation. It was noticed that the HTC of water and water-TiO2 nanofluid enhanced with the enhancement in Reynolds number and pressure. HTC of water-TiO2 nanofluid was greater than the water and enhanced with the enhancement in volumetric concentration. The highest increment in the HTC of water-TiO2 nanofluid in comparison to water was noticed for Reynolds number 24,000, concentration 0.20%, and pressure 2.5 bar which was 76.7%. Correlations of the HTC of titania nanofluid were developed. To optimize the HTC of water-TiO2 nanofluid desirability function approach was utilized.

Analysis of the Multivalued Measure of Thermal Conductivity

We analyze a new class of instruments aimed at measuring heat quantities based on the use of a multivalued measure of thermal conductivity of solid bodies. On the example of measuring of the thermal conductivity of solids, we illustrate the incorrectness of application of the principle of multivaluedness for the measures of intensive heat quantities. To prove this fact, we present relations for the thermal conductivities of elements of a heat sensor realizing a multivalued measure of thermal conductivity and perform the limit transition in these relations. By using two methods, we show that the thermal conductivity of the indicated measure does not depend on the value of the supplied heat flux. It is shown that the accuracy of the method of measurements of thermal conductivity and specific heat capacity with the use of multivalued measures disagrees with the actually attainable levels of accuracy and the accuracy of reproduction of the unit of surface density of heat flux with the help of the GET 172-2016 State Primary Standard. The currently attainable accuracy of measurements of the thermal conductivity of solids is estimated.

Thermal Performance of Constrained Melting of PCM Inside an Elliptical Capsule of Two Orientations

The phase change material is deemed a hopeful method of saving a large quantity of thermal energy latently at an approximately low-temperature swing. A computational investigation to examine the thermal behavior of constrained phase change of paraffin wax inside an elliptical cell under the influence of different wall heat fluxes is presented. The impact of two orientations of the capsule (prolate and oblate) on the transient movement of the melting-front and liquid fraction is examined. The results exhibit that the imposed wall heat flux has an important influence on the melting characteristics, whereas a small effect on the melting process is exhibited by the orientation of the capsule. Nevertheless, the melting process is relatively faster and lower time is required in an oblate capsule than that experienced in the prolate capsule. The reduction in fusion time resulted from container orientation increases with the increase in supplied heat flux. The maximum reduction in the melting time of the oblate capsule is about 7.71% less than that of the prolate capsule for the same size. Besides, it is found that the time needed for complete melting in an elliptical cell is lower than that observed in spherical one for the same size, surface area and amount of heat flux.

Experimental feasibility study of a new attached hydronic loop design for geothermal heating of bridge decks

Geothermally heating of bridges is a green and sustainable alternative to deicing chemicals, which are energy-intensive, corrosive to the bridges, and dangerous to the environment. Geothermal bridges harness shallow earth geothermal energy, a readily available all-year-round renewable energy source. However, the existing geothermal heated bridge technology was developed based on embedded hydronic loops, which can be only deployed during the construction of new bridges. As there are more existing bridges in the critical need of deicing, a new attached hydronic loop design was developed in this study for geothermal heating of existing bridges. The hydronic loops are attached to the bottom of a bridge deck and are encapsulated inside an insulation material geofoam. The developed hydronic loop was tested on a lab-scale concrete deck with supplied warm water to evaluate its feasibility for field application. This heated deck system was fabricated and instrumented for the heating performance tests in an environmental chamber. It was first tested under various room temperatures higher than 4.4 °C, the lowest temperature permitted by the environmental chamber. Based on the test results, an empirical prediction equation is presented to estimate deck temperature at given ambient and supplied heating fluid temperatures. The applicability of this equation to freezing temperature was validated in localized freezing tests by placing a cooling box on the test deck. All test results show that this heated bridge deck allows efficient heat transfer, approximately 60% of the supplied heat, to the bridge deck surface. The supplied heat flux to the deck surface ranges from 120 to 270 W/m2 depending on the ambient and heating fluid temperature, which is comparable to the embedded hydronic system. The new hydronic loop design is feasible to be implemented on the existing bridges for deicing and snow removal.

Comparing the integral characteristics of secondary droplet atomization under different situations

In this paper, we describe the results of experimental research into secondary droplet atomization for several heterogeneous, water-oily liquid compositions including some highly heterogeneous emulsions and slurries. The group of schemes is studied, in addition to relevant experiments reported in the literature: droplets colliding with each other, with a heated or not heated walls, and with an air flow, as well as exposed to conductive or convective heating followed by micro-explosive breakup. After the analysis of experimental data, we calculate the size and number distributions of the liquid fragments generated using each of the approaches. We determine the duration of initial droplets' fragmentation as a function of the We numbers and the density of the supplied heat flux. The comparative analysis gave us optimal conditions for several aerosol generation techniques providing minimal fragment size with relatively low electric power consumption. It is shown that each of used atomization techniques provides a comparable number of liquid fragments on average, considering the amount of power required. Several techniques for combined atomization are proposed, based on the investigated approaches.

Investigation of the evaporation heat transfer mechanism of a non-axisymmetric droplet confined on a heated micropillar structure

In scaling computer performance, closely integrating heterogeneous components in a single system-in-package assembly provides faster signal communication and a tighter footprint – but also generates more heat: state-of-the-art microelectronic devices can produce more than 1 kW cm−2. The next generation high-powered electronics will need two-phase liquid cooling, such as droplet evaporation, which utilizes the latent heat of vaporization to remove excessive heat. Compared to traditional single-phase cooling techniques, two-phase cooling offers both high efficiency and an exceptionally high heat dissipation rate. Although droplet evaporation has been explored for over a hundred years, many fundamental transport mechanisms are still not well understood. For example, most droplet evaporation studies focused only on spherical droplets, which possess a uniform curvature, κ (for a low Bond Number), along the meniscus interface. Evaporation from a non-spherical droplet, due to the change in perimeter-to-area ratio and the meniscus curvature, exhibits very different interfacial mass transport features from a spherical droplet. In particular, a higher perimeter-to-solid-liquid-area ratio will yield a larger fraction of thin film region and therefore a smaller thermal resistance in an evaporating droplet, while a high local curvature will facilitate a stronger local vapor diffusion rate. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on heated porous micropillar structures. We explore the equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes, using the Volume of Fluid (VOF) method, and we use a simplified Schrage model [1] to study the evaporative mass transport at the liquid-vapor interface. The results show that microdroplets evaporating on a triangular substrate possess 12.8% smaller effective film thickness compared to that on a circular substrate, due to a longer length of the contact line. During the evaporative heat transfer process, the triangular-based droplets also exhibit the smallest temperature difference between the droplets’ solid-liquid interface and ambient temperature, which leads to a higher heat transfer coefficient (21% larger than a spherical droplet at a supplied heat flux of 500 W/cm2). When the supplied heat increases to a higher value (e.g., 1000 W/cm2), the shape effect becomes less significant where the diffusion resistance is dominated by the liquid-vapor interfacial temperature instead of the meniscus curvature.

Experimental investigations on heat transfer enhancement in shell coil heat exchanger with variable baffles geometry

The paper presents the possibility of using passive intensification of heat transfer in the form of baffles to increase the energy efficiency of the shell and coil heat exchanger. The experiment was carried out by using a modular coil heat exchanger in the form of an electric heater. Water was used as a working fluid with constant thermal-flow parameters at the inlet of the module. It should be noted that experiments were made for a large range of power i.e. from 200 W to 1200 W and with mass flow rates from 0.01 kg−1 to 0.03 kg−1. For this flow range, Reynolds numbers were obtained in the field 150 < Re < 450. Consequently, all experiments were carried out for laminar flow conditions. This work shows, that due to the presence of mixed convection, natural convection has a significant effect on small values of Reynolds numbers and large values of Richard’ number. Also, the baffles location has a significant influence on HX performance. The proposed solution of the shell coil exchanger with a baffle is characterized by better heat transfer efficiency on the shell side, but rather for lower values of Reynolds (Re < 150) numbers and high values of heat fluxes. For all configurations is noticeable the influence of the supplied heat flux value on the rate of heat transfer. The paper presents new experimental Nusselt numbers correlation on the shell side of the heat exchanger with core-baffles. This correlation depends on Grashoff and Dean numbers as well as dimensionless shell diameter (C/D0), the correlation also includes Prandtl number. Experimental data were compared with selected correlations from literature as well as with own correlation. It was shown that only own semi-empirical correlation has satisfactory compliance with experimental results within 35% error band and with less than 8% of absolute deviation. The considered construction has markedly better heat transfer efficiency on the shell side for small Reynolds numbers (Re<150) comparing to the tube-in-tube exchanger with wire coil turbulator(over 20%).

Influence of different heating types on the pumping performance of a bubble pump

This study presents an experimental investigation of the influence of different heating types on the pumping performance of a bubble pump. A test rig was set up at the Institute of Thermodynamics and Thermal Engineering (ITW), University of Stuttgart. The vertical lift tube is made of copper with an inner diameter of 8 mm and a length of 1.91 m. The working fluid is demineralized water. The test rig offers the possibility to vary the supplied heat flow (0 W − 750 W), the resulting supplied heat flux and the location of the heating. Investigations were carried out using spot heating, partial-length heating and full-length heating. A Coriolis mass flowmeter was successfully implemented which measures the vapor mass flow rate continuously. The improvement of the vapor mass flow rate measurement by using the continuous measurement method compared to a discontinuous one is discussed. Furthermore, the influence of an unstable inlet temperature of the working fluid entering the lift tube on the pumping performance is investigated. The focus of this publication lies on the build-up of the test rig with the measurement setup and the analysis of the pumping performance for the three heating types. The measurement results show a big influence of the heating type on the pumping performance. The lower the relative length of the heating, the higher is the pumping ratio which is defined as the lifted liquid mass flow rate in relation to the generated vapor mass flow rate.

Heat rate distribution in converging and diverging microchannel in presence of conjugate effect

This study pertains to understanding the effect of wall conduction on heat rate distribution through varying cross sectional area microchannel. Three-dimensional numerical simulations are performed for flow in diverging and converging microchannels with different geometrical configurations (angle: 1–8°, depth: 86–200μm, length: 10–30mm, solid-to-fluid thickness ratio: 1.5–4), thermo-physical properties (solid-to-fluid thermal conductivity ratio: 27–646), mass flow rate (3.3×10−5–8.3×10−5kg/s) and heat flux (2.4–9.6W/cm2) conditions. It is observed that area variation and wall conduction leads to a redistribution of the supplied heat flux particularly along the flow direction. It is observed that wall conduction in converging microchannel (M=0.028–0.064, where M is the wall conduction number) is much larger than in diverging microchannel (M=0.006–0.024). For converging microchannel, wall conduction varies directly with the depth and solid-to-fluid thickness and conductivity ratio; whereas it exhibits inverse function with the angle, length and Reynolds number. Similarly, in diverging microchannel, wall conduction depends on all these parameters. The variation in heat rate versus surface temperature gradient exhibits two separate zones demarcating diverging and converging microchannels. The variation in heat rate is found to be well-correlated to the wall conduction number for most cases. The study is relevant because of increased application of varying cross section micro devices, and the present results should help in their thermal design.