Heat Flux Probe Research Articles

Heat transfer efficiency in gas–solid fluidized beds with flat and corrugated walls

Abstract Gas–solid fluidized bed reactors exhibit improved heat and mass transfer performance as compared to packed beds. Corrugated walls installed in narrow gas–solid bubbling fluidized bed (CWBFB) enclosures have been observed to decrease minimum bubbling velocity, reduce bubble size, improve gas distribution, provide stable operation, and minimize particle carryover or loss. Thorough analyses of the wall-to-bed heat transfer coefficient in flat- (FWBFB) and corrugated- (CWBFB) wall bubbling fluidized beds have been performed for a variety of operating conditions and geometric parameters. Fast-response self-adhesive heat flux probes and thermocouples were used to simultaneously measure the wall-to-bed heat flux, surface and bed temperatures, and were used to determine the heat transfer coefficient (HTC) at various axial and lateral locations. For a given set of parameters, a significant increase in HTC was observed at lower gas flow rates in CWBFB as compared to FWBFB. It was shown that CWBFB inventory required lower U mb (gas flow rate) as compared to FWBFB. Full 3-D transient Euler–Euler CFD simulations using the kinetic theory of granular flow were also performed, which confirmed the experimental results.

Methods and metrics to assess the accuracy of heat flux data from a spark ignition IC engine

The measurement of in-cylinder heat transfer can be a valuable diagnostic tool for quantifying and improving IC engine performance. Increased heat transfer out of the combustion chamber negatively impacts overall efficiency, while heat transfer from the metal to the charge can increase knock propensity. The heat flux at various locations in an engine cylinder can be measured using heat flux probes consisting of two thermocouples – a surface thermocouple exposed to the changing in-cylinder temperatures, and a far field thermocouple which has a constant temperature at steady state operating conditions. The buildup of carbon deposits on the surface thermocouple can affect the heat flux measurements and lead to errors in the data. Heat flux measurements were performed on a proprietary single-cylinder research engine with instrumented heat flux probes in the cylinder head and cylinder liner. The engine was run rich with a high PMI fuel to expedite the buildup of carbon deposits. A metric was developed based on the output of the surface thermocouple to determine the cleanliness of the heat flux probes non-intrusively. Heat flux calculations were done using the FFT method and validated using the Cook-Felderman method. An uncertainty analysis was conducted on the heat flux data to verify that the negative heat flux observed was not due to errors in the distance between the two thermocouples or the measurement of temperature. Finally, to bolster confidence in the results, heat flux measurements were made for motoring conditions with varying coolant and intake air temperatures to gauge whether the trends in the output were in the expected direction.

Analysis of combustion chamber deposit growth on temperature swing thermal barrier coatings in a spark ignition engine

Temperature swing thermal barrier coatings (TBCs) have the potential to improve efficiency and performance through thermal management of the combustion chamber. However, the effects of combustion chamber deposits (CCDs) on the surface of the TBCs are unclear. Therefore, this paper analyzes the impact of CCD formation on a temperature swing TBC. A piston and a heat flux probe coated with a novel material are installed in a single-cylinder research engine at a low-load condition for 62.5 h to promote CCD growth. Every 12.5 h, the performance at a knock limited condition is assessed and thermophysical property measurements on the heat flux probe are performed. Net thermal efficiency increased by 0.4% absolute after 12.5 h relative to the baseline condition, but further CCD growth caused a dithering of efficiency between the 12.5 h and baseline points. The KLSA retarded consistently throughout this period. External property measurements with the coated heat flux probe showed an improvement in the thermophysical properties of the TBC/CCD layer.

A method to quantify the supersonic discharge of airbag cold gas inflators

We discuss a method to quantify the supersonic discharge of airbag cold gas inflators. Since one primary quantity of interest, the mass flow with time, is not directly measurable, a combined experimental and numerical approach was chosen. Shadowgraph and schlieren images visualize the gas dynamic process. Pressure measurements were conducted inside the inflator and downstream of the outlets in the supersonic jet. In this context, a method to measure the pressure of the flow without effects of shock reflection is presented. The temperature inside the inflator was estimated using a fast response heat flux probe and assuming different scenarios for the unknown heat transfer coefficient. Then, a numerical model of the inflator was created. The experimental results served as boundary conditions and some basic sensitivities remaining unknown from the measurements were studied to verify the numerical outcome. The numerical model was verified using experimental results. Finally, the mass flow rate was derived from the numerical model and compared to an analytical model. The method can reconstruct the temporal evolution of the mass flow discharging from the inflator, the pressure and the topology of the flow field within acceptable bounds. Furthermore, the method can deliver inflow data for subsequent airbag inflation studies.Graphical abstract

Estimating the power injection proportion of the plasma jet generator based on the measured enthalpy

The purpose of the developed plasma jet generator is to study the effect of jet plasma on electromagnetic wave propagation, simulating the plasma sheath environment encountered during space vehicle reentry. Because the temperature and velocity of the plasma jet are high, it is difficult to measure temperature and electron density by the contact method. In addition to the normal flow of the entire apparatus, high-power magnetic field energy (generated by the power supply) is injected into the inductively coupled plasma (ICP) generation region. The energy of the magnetic field excites the gas and generates plasma. In this paper, the whole apparatus is calculated numerically, the energy term is added to the corresponding area of ICP, and the enthalpy distribution of the whole apparatus is calculated. The heat flux and total pressure are measured by slug calorimeter (heat flux probe) and total pressure probe, and then, the enthalpy is obtained. When the calculated enthalpy is consistent with the measured enthalpy, the value of the energy term in the numerical calculation process is the injected energy value. The ratio of the energy injected into the ICP region to the total energy provided by the power supply is the power injection proportion. The power injection proportion of the apparatus in this paper obtained by the above method is between 25% and 30%.

Flow morphology and heat transfer analysis during high-pressure steam condensation in an inclined tube part I: Experimental investigations

In this paper, experimental investigations on the flow morphology and heat transfer in a single steam condenser tube are presented, which were performed at the thermal hydraulic test facility COSMEA (COndensation test rig for flow Morphology and hEAt transfer studies). This facility was setup to study the interrelation of condensation heat transfer with two-phase flow in a single condenser tube that is cooled by forced convection. Studies were performed for elevated pressures up to 65 bar at saturation conditions and for inlet steam mass flow of up to 1 kg/s and different inlet steam qualities. The wall heat flux is measured with distributed heat flux probe and global condensation rates were obtained from integral heat and mass balances. As a unique feature the cross-sectional phase distribution was studied via X-ray computed tomography. The data is going to be used for the validation of numerical simulations with 1D ATHLET and 3D CFD codes as presented in the second part of this paper.

Combustion Induced Choking and Unstart Initiation in a Circular Constant-Area Supersonic Flow

Incipient choking and unstart initiation mechanisms are investigated experimentally in an ethylene-fueled constant-area direct-connect supersonic combustor with a circular cross section (diameter of 28 mm) and full optical access. The experiments are performed in the ACT-II pulsed arc-heated hypersonic wind tunnel of the University of Illinois at Urbana–Champaign. The extended test time (up to 1 s) and the visualization capabilities allowed by the circular geometry represent major novelties as compared to previous studies. The combination of intrusive (pressure sensors, pitot tube, heat flux probe) and nonintrusive (planar laser-induced fluorescence of the hydroxyl radical) diagnostics provides unprecedented details on the flow behavior near the choking limit. Parallel tests with fuel and air injection are conducted to isolate the effects of combustion from mass injection. It is observed that combustion produces minimal heat addition in the core flow and that the induced pressure gradient is not sufficiently strong to separate the boundary layer according to the Korkegi criterion. Combustion is found to interact with the supersonic core flow mainly through the boundary layer, where most of the combustion and heat release occurs. Stagnation pressure losses across the combustor and Mach number at the exit are well correlated by Fanno flow theory, indicating a predominant role of irreversibilities on the reduction of the Mach number that ultimately leads to choking.

Development of a Continuous Fluidized Bed Reactor for Thermochemical Energy Storage Application

Thermochemical energy storage (TCES) represents one of the most promising energy storage technologies, currently investigated. It uses the heat of reaction of reversible reaction systems and stands out due to the high energy density of its storage materials combined with the possibility of long-term storage with little to no heat losses. Gas–solid reactions, in particular the reaction systems CaCO3/CaO, CaO/Ca(OH)2 and MgO/Mg(OH)2 are of key interest in current research. Until now, fixed bed reactors are the state of the art for TCES systems. However, fluidized bed reactors offer significant advantages for scale-up of the system: the improved heat and mass transfer allows for higher charging/discharging power, whereas the favorable, continuous operation mode enables a decoupling of storage power and capacity. Even though gas–solid fluidized beds are being deployed for wide range of industrial operations, the fluidization of cohesive materials, such as the aforementioned metal oxides/hydroxides, still represents a sparsely investigated field. The consequent lack of knowledge of physical, chemical, and technical parameters of the processes on hand is currently a hindering aspect for a proper design and scale-up of fluidized bed reactors for MW applications of TCES. Therefore, the experimental research at Technical University of Munich (TUM) focuses on a comprehensive approach to address this problem. Preliminary experimental work has been carried out on a fixed bed reactor to cover the topic of chemical cycle stability of storage materials. In order to investigate the fluidization behavior of the bulk material, a fluidized bed cold model containing a heat flux probe and operating at atmospheric conditions has been deployed. The experimental results have identified the heat input and output as the most influential aspect for both the operation and a possible scale-up of such a TCES system. The decisive parameter for the heat input and output is the heat transfer coefficient between immersed heat exchangers and the fluidized bed. This coefficient strongly depends on the quality of fluidization, which in turn is directly related to the geometry of the gas distributor plate. At TUM, a state-of-the-art pilot fluidized bed reactor is being commissioned to further investigate the aforementioned aspects. This reactor possesses an overall volume of 100 L with the expanded bed volume taking up 30 L. Two radiation furnaces (64 kW) are used to heat the reactor. The heat of reaction of the exothermal hydration reaction is removed by water, evaporating in a cooling coil, immersed in the fluidized bed. Fluidization is being achieved with a mixture of steam and nitrogen at operating temperatures of up to 700 °C and operating pressures between −1 and 6 bar(g). The particle size is in the range of d50 = 20 μm. While initial experiments on this reactor focus on optimal operating and material parameters, the long-term goal is to establish correlations for model design and scale-up purposes.

Computational Predictions of the Hypersonic Material Environmental Test System Arcjet Facility

The Hypersonic Materials Environmental Test System arcjet facility located at the NASA Langley Research Center in Hampton, Virginia, is primarily used for the research, development, and evaluation of high-temperature thermal protection systems for hypersonic vehicles and reentry systems. To improve testing capabilities and knowledge of the test article environment, a detailed three-dimensional model of the arcjet nozzle and the freejet portion of the flowfield is developed. The computational fluid dynamics model takes into account nonuniform inflow state profiles at the nozzle inlet as well as catalytic recombination efficiency effects at the probe surface. The results of the numerical simulations are compared to the calibrated pitot pressure and the stagnation-point heat flux for three test conditions at low, medium, and high enthalpies. Comparing the results and the test data indicates a partially catalytic copper surface on the heat flux probe of about 10% recombination efficiency and a 2–3 kPa pressure drop from the total pressure measured at the plenum section in front of the nozzle. With these assumptions, the predictions are within the uncertainty of the stagnation pressure and heat flux measurements. The predicted velocity conditions at the nozzle exit are also compared and show good agreement with the radial and axial velocimetry data.

Measurement of the aerothermodynamic state in a high enthalpy plasma wind-tunnel flow

This paper presents spatially resolved measurements of absolute particle densities of N2, N2+, N, O, N+, O+, e− and excitation temperatures of electronic, rotational and vibrational modes of an air plasma free stream. All results are based on optical emission spectroscopy data. The measured parameters are combined to determine the local mass–specific enthalpy of the free stream. The analysis of the radiative transport, relative and absolute intensities, and spectral shape is used to determine various thermochemical parameters. The model uncertainty of each analysis method is assessed. The plasma flow is shown to be close to equilibrium. The strongest deviations from equilibrium occur for N, N+ and N2+ number densities in the free stream. Additional measurements of the local mass-specific enthalpy are conducted using a mass injection probe as well as a heat flux and total pressure probe. The agreement between all methods of enthalpy determination is good.

Unsteady simulation of hypersonic flow around a heat flux probe in ground testing conditions

In order to investigate the accuracy of the rebuilding code for the free stream conditions and the total enthalpy in the Longshot Hypersonic facility at the von Karman Institute (VKI), a series of unsteady CFD simulations of axisymmetric hypersonic flow over a heat flux probe have been performed. The free stream and the total conditions provided by experiments as a function of time were used as unsteady inlet boundary conditions for those simulations. The full duration (15ms) of the test window is simulated by means of an implicit time accurate finite volume solver. The numerical method and the time integration procedure used during this investigation are described. The numerical results are within 6% of the experimental data for both the stagnation pressure and the heat flux at the reference test time conditions.

Investigations of flow structure and liquid mixing in bubble column equipped with selected internals

Flow structures and liquid mixing effects are examined in a gas sparged column equipped with internals selected to achieve desired effects. The internals used in this study include a concentric tube bundle, a baffle designed to alter flow of bubbles across column cross-section and a multi-arm gas distributor with downward facing holes. The selection represents consideration of heat removal system, minimal obstruction to flow and desirable mixing effects. Measurements have been made in a 0.15m ID column in two-phase air-water and three phase air-water-glass beads systems. The air flow rate is varied from 0.03 to 0.3m/s to cover a wide range of velocities and the particles are 40µm glass beads. Liquid phase mixing effects are examined with tracer injections from which mixing time and axial dispersion coefficients are obtained. For more detailed survey of flow structure neutrally buoyant particles are employed. Measurements with a fast response heat flux probe in two and three phase systems are compared and analyzed to demonstrate its potential use for screening of internals. Averaged flow structure in the presence of the internals is proposed from observations with different techniques.

Evaluation of a new device for simultaneous measurement of heat flux and gas velocity in a diffusion flame

This article examines potential use of a new device called multi-directional heat flux and velocity probe for simultaneous measurement of heat flux and flame speed in a diffusion flame. The probe consists of a thin-wall spherical shell with internal insulation to mitigate internal convection. Both pressure and temperature distributions around the sphere are used to indicate local velocity and heat flux. The multi-directional heat flux and velocity probe appears to be a more promising device than the bidirectional velocity probe in the sense that the sphere is a regular geometry with minimum flow separation and should lead to more predictable behavior. However, an outcome of this study is that the device must be used in conjunction with a fire code computational fluid dynamics model because the boundary layer is not isothermal so that the conventional pressure coefficient for a sphere leads to erroneous results.

Teflon probing for the flow characterization of arc-heated wind tunnel facilities

The experimental flow characterization of the arc-heated wind tunnel of the University of Texas at Arlington is investigated in this work using ablative Teflon probes in combination with total pressure measurements. A parallel analytical work, focused on the dimensional analysis of the ablation process, has been conducted with the purpose of improving existing semi-empirical correlations for the heat blockage due to the mass injection inside the boundary layer. A control volume analysis at the receding surface of the specimens is used to calculate the wall heat transfer for a non-ablating probe by including the blockage effect. The new correlations, obtained for the convective blockage, show an improvement of the correlation coefficient of 110 % with respect to those available in literature, once a new blowing parameter containing the stagnation pressure is introduced. A correlation developed by NASA during the Round-Robin program, which relates the Teflon mass loss rate to the total pressure and cold-wall heat flux measured experimentally, is also used to predict the wall heat transfer referred to the ablation temperature of Teflon. For both approaches, a simplified stagnation point convective heat transfer equation allows the average stagnation enthalpy to be calculated. Several locations downstream of the nozzle exit have been surveyed, and selected points of the facility’s performance map have been used for the experimental campaign. The results show that both approaches provide similar results in terms of stagnation heat flux and enthalpy prediction with uncertainties comparable to those provided by standard intrusive heat flux probes (δ q max < 25 %). The analysis of the Teflon’s ablated surface does not reveal significant flow non-uniformities, and a 1.14 heat flux enhancement factor due to the shock–shock interaction is detectable at x = 3.5 in. from the nozzle exit plane. The results show the use of ablative probes for the flow characterization of arc plasma facilities to be promising for the dual purpose of calculating the local flow properties (i.e., heat flux and enthalpy) as well as verifying the uniformity of the flow by inspecting the footprint of the plume on the exposed surfaces.

Heat Transfer and Turbulent Fluid Flow over Vertical Double Forward-Facing Step

Abstract — Numerical study of heat transfer and fluid flow over vertical double forward facing step were presented. The k-w model with finite volume method was employed to solve continuity, momentum, and energy equations. Different step heights were adopted for range of Reynolds number varied from 10000 to 40000, and range of temperature varied from 310K to 340 K. The straight side of duct is insulated while the side of double forward facing step is heated. The result shows augmentation of heat transfer due to the recirculation region created after and before steps. Effect of step length and Reynolds number observed on increase of local Nusselt number particularly at recirculation regions. Contour of streamline velocity is plotted to show recirculation regions after and before steps. Numerical simulation in this paper done by used ANSYS FLUENT 14. Keywords — Turbulent flow, Double forward, Heat transfer, Separation flow.I. I NTRODUCTION HE crises of energy in global has been encourage researchers to look for new techniques which improve of thermal performance. One of common method to increase efficiency of energy system equipment is by change the design geometry of channel. The fluid flow over forward or backward-facing step found in many practical applications such as heat exchangers, chemical process, turbine blades, and power plants. There are many experimental and numerical study of effect separation flow at forward or backward-facing step on increase of enhancement heat transfer rate. Heat transfer to fluid flow over single and double forward-facing step experimentally studied by Shakouchi and Kajino [1] with using laser Doppler anemometer where the results appeared that the reattachment region by a new heat flux probe, and Oyakawa et al. [2], [3] used jet discharge. Saldana and Anand [4] performed numerical study of laminar fluid flow over a horizontal forward-facing step in three dimensions. They showed that the increase of Reynolds number leads to increase of reattachment length and average Nusselt number.

Effective thermal conductivity of hollow bricks with cavities filled by air and expanded polystyrene

Effective thermal conductivity of hollow bricks with the cavities filled by either air or expanded polystyrene is analyzed using a hybrid experimental–computational approach. The experimental setup involves an application of a thermal insulation box and a set of temperature and heat flux probes placed at characteristic positions of the specimen-insulating box system. Using the measured heat fluxes and temperatures, the heat loss of the system is determined. A computer simulation tool based on the finite element principle is then used for modeling the temperature fields and heat losses in the studied system. Finally, the effective thermal conductivity is identified using an iterative procedure. Experimental results show that the application of expanded polystyrene as cavity filler instead of air leads to ∼30% decrease in the effective thermal conductivity of hollow brick blocks.

Scanning ion sensitive probe for plasma profile measurements in the boundary of the Alcator C-Mod tokamak

A new Ion Sensitive Probe head has been created for the outer-midplane scanning probe system on the Alcator C-Mod tokamak. The new probe head contains three elements: an ion sensitive probe to measure ion temperature and plasma potential, a Langmuir probe to measure electron temperature, density, and floating potential, and a second Langmuir probe to measure ion saturation current and the density fluctuations arising from ''blob'' events. The ion sensitive probe current is normalized to this measurement to reduced deleterious effects of the strong fluctuations. Design of the high heat flux probe (>100 MW/m(2)) and initial results are presented.

High-Enthalpy Plasma Facility for Fusion Plasma Material Interaction Studies

A high-enthalpy plasma facility with 0.4 MW power level has been constructed at Chonbuk National University in Korea. A segmented arc plasma torch was adopted as a plasma source for this facility. It is designed to have high thermal efficiency and long life in the production of supersonic plasma flow with enthalpy above 13 MJ/kg at a velocity of Mach 3. Diagnostic systems include enthalpy probe with mass spectrometer for the measurement of enthalpy, plasma composition and plasma temperatures, and heat flux probe for the measurement of heat flux. Visualization systems such as a pyrometer, a fast camera and an emission spectroscopy have been installed to monitor the surface temperature of the substrate, the plasma density and flow patterns.

In-furnace temperature and heat flux mapping in a kraft recovery boiler

Gas temperatures, incident heat flux, and surface temperatures were measured in a large kraft recovery boiler. The measurements were a part of an extensive campaign planned and carried out to support validation of models based on computational fluid dynamics. The gas temperatures were measured with three different techniques: infrared (IR) pyrometer, suction pyrometer, and unshielded thermocouples. In addition to the temperature measurements, the radiative heat flux was measured in a number of locations in the boiler using a portable heat flux probe, and the surface temperatures inside the boiler were measured using a portable single-band IR camera.

Literature Review on Heat Transfer in Two- and Three-Phase Bubble Columns

Fischer-Tropsch synthesis (FTS), an exothermic reaction where hydrogen and carbon monoxide synthesis gas are converted to hydrocarbon products, has been under development since the 1930's. The interest in FTS depends on current and perceived future prices of crude oil but is increasingly viewed as an option for exploiting stranded natural gas. Other advantages of FTS hydrocarbons include the absence of sulphur, nitrogen, heavy metal contaminants, low aromatic content and the ability to produce high value middle distillates/fuels. Current interest is directed towards slurry bubble processes – comprising gas, liquid, and solid phases. Industrial slurry phase FTS reactors may range in size from 6 – 10 m in diameter and upwards of 30 m in height and include multiple internal heat transfer tubes. Such systems offer numerous advantages including high heat transfer rates, good mixing, and ease of online catalyst addition and withdrawal. However, one disadvantage is the complex hydrodynamics associated with slurry bubble columns, which make scale-up difficult. A literature review on heat transfer studies and correlations has been completed focusing on previous experimental setups, the synthesis of the key findings/parameters, and the identification of the necessary criteria required for reactor design and scale-up.The parameters having the most pronounced impact on heat transfer in slurry bubble columns and three-phase fluidized beds are the superficial gas velocity and liquid properties such as viscosity and surface tension, which significantly alter the bubble properties and the column hydrodynamics. The effect of particles is poorly understood and is a complex function of particle diameter and concentration. The experimental results and correlations reported here from the majority of studies are dependent upon the equipment and properties of the three phases studied – resulting in very limited applicability to other systems or for scale-up. Other concerns include the use of relatively low gas velocities, ambient temperature and pressure, relatively large particles, and relatively small columns employed in most studies, which are not relevant to industrial operating conditions. Furthermore, studies involving multiple internals were relatively few. Most columns were only equipped with a single tube or small heat flux probe thereby measuring only the local heat transfer and not taking into account the effect on column hydrodynamics of multiple internals. Of these studies only a few tubes were equipped with heaters (that did not run the entire tube length) and heat flux probes while the remaining probes were inactive.