Adiabatic Heat Transfer Research Articles

Heat transfer and pressure drop of R32 evaporating in one pass microchannel tube with parallel channels

This paper presents heat transfer coefficient and pressure drop of R32 measured in the same facility as introduced in Li and Hrnjak (2017). Experiments are conducted on a 24-port microchannel tube with a hydraulic diameter of 0.643 mm. The pressure drop of R32 is lower than R134a, and the heat transfer coefficient is higher at the same condition. The pressure drop increases with quality and mass flux but decreases with reduction of saturation pressure. R32 heat transfer coefficient increases when heat or mass flux rises. Heat transfer coefficient of R32 first increases when quality increases due to convective effects and then drops at moderate quality due to the absence of nucleate boiling and dry-out. A comparison of adiabatic pressure drop and heat transfer coefficient to correlations has been made. Based on the Blasius equation and homogeneous density, the viscosity model from Cicchitti et al. (1960) has the best fit to predict two-phase pressure drop. Kim and Mudawar (2012) has the smallest MAE to predict pressure drop. For heat transfer coefficient, Sun and Mishima (2009) is the best fit. Further research will be on expanding the database and understanding hydraulic and boiling behavior.

Analysis of adiabatic heat and mass transfer of microporous hydrophobic hollow fiber membrane-based generator in vapor absorption refrigeration system

A microporous hydrophobic hollow fiber membrane-based generator (HFM-G) is expected to be an alternative type of generator in vapor absorption refrigeration system, which can be compact and lightweight with the enhanced heat and mass transfer. This paper pursues a comprehensive understanding of a detailed desorption mechanism of the proposed HFM-G, and suggests the feasible configuration for the practical use. A theoretical HFM-G mechanism is first introduced to gain an understanding of the adiabatic heat and mass transfer. A gas permeation test determines the nominal pore size of the membrane, which characterizes the mass transfer across the membrane. Transient experiments demonstrate the characteristics of the adiabatic desorption process on the HFM-G for wide range of feed solution concentrations under various practical operating conditions. A comparison shows that the experimental heat and mass transfer results are consistent with the theoretical values of adiabatic desorption heat and mass transfer on the HFM-G.

Effects of Force Fields on Interface Dynamics, in view of Two-Phase Heat Transfer Enhancement and Phase Management for Space Applications

On earth, gravity barely influences the dynamics of interfaces. For what concerns bubbles, buoyancy governs the dynamics of boiling mechanism and thus affects boiling heat transfer capacity. While, for droplets, the coupled effects of wettability and gravity affects interface exchanges. In space, in the lack of gravity, rules are changed and new phenomena come into play. The present work is aimed to study the effects of electric field on the shape and behaviour of bubbles and droplets in order to understand how to handle microgravity applications; in particular, the replacement of gravity with electric field and their coupled effects are evaluated. The experiments spread over different setups, gravity conditions, working fluids, interface conditions. Droplets and bubbles have been analysed with and without electric field, with and without (adiabatic) heat and mass transfer across the interface. Furthermore, the results of the 4 ESA Parabolic Flight Campaigns (PFC 58, 60, 64 & 66), for adiabatic bubbles, adiabatic droplets and evaporating droplets, will be summarized, discussed, and compared with the ground tests.

Assessment of modelling strategies for film cooling

PurposeEffusion cooling represents one the most innovative techniques for the thermal management of aero-engine combustors liners. The huge amount of micro-perforations implies a significant computational cost if cooling holes are included in computational fluid dynamics (CFD) simulations; therefore, many efforts are reported in literature to develop lower-order approaches aiming at limiting the number of mesh elements. This paper aims to report a numerical investigation for validating two approaches for modelling film cooling, distinguished according to the way coolant is injected (i.e. through either point or distributed mass sources).Design/methodology/approachThe approaches are validated against experimental data in terms of adiabatic effectiveness and heat transfer coefficient distributions obtained for effusion cooled flat plates. Additional reynolds-averaged naver stokes (RANS) simulations were performed meshing also the perforation, so as to distinguish the contribution of injection modelling with respect to intrinsic limitations of turbulence model modelling.FindingsDespite the simplified strategies for coolant injection, this work clearly shows the feasibility of obtaining a sufficiently accurate reproduction of coolant protection in conjunction with a significant saving in terms of computational cost.Practical/implicationsThe proposed methodologies allow to take into account the presence of film cooling in simulations of devices characterized by a huge number of holes.Originality/valueThis activity represents the first thorough and quantitative comparison between two approaches for film cooling modelling, highlighting the advantages involved in their application.

Adiabatic wall temperature and heat transfer coefficient influenced by separated supersonic flow

Investigations of supersonic air flow around plane surface behind a rib perpendicular to the flow direction are performed. Research was carried out for free stream Mach number 2.25 and turbulent flow regime - Rex >2·107 . Rib height was varied in range from 2 to 8 mm while boundary layer thickness at the nozzle exit section was about 6 mm. As a result adiabatic wall temperature and heat transfer coefficient are obtained for flow around plane surface behind a rib incontrast with the flow around plane surface without any disturbances.

Film Cooling Parameter Waveforms on a Turbine Blade Leading Edge Model With Oscillating Stagnation Line

It is necessary to understand how film cooling influences the external convective boundary condition involving both the adiabatic wall temperature and the heat transfer coefficient in order to predict the thermal durability of a gas turbine hot gas path component. Most studies in the past have considered only steady flow, but studies of the unsteadiness naturally present in turbine flow have become more prevalent. One source of unsteadiness is wake passage from upstream components which can cause fluctuations in the stagnation location on turbine airfoils. This in turn causes unsteadiness in the behavior of the leading edge coolant jets and thus fluctuations in both the adiabatic effectiveness and heat transfer coefficient. The dynamics of h and η are now quantifiable with modern inverse heat transfer methods and nonintrusive infrared thermography. The present study involved the application of a novel inverse heat transfer methodology to determine time-resolved adiabatic effectiveness and heat transfer coefficient waveforms on a simulated turbine blade leading edge with an oscillating stagnation position. The leading edge geometry was simulated with a circular cylinder with a coolant hole located 21.5 deg downstream from the leading edge stagnation line, angled 20 deg to the surface and 90 deg to the streamwise direction. The coolant plume is shown to shift in response to the stagnation line movement. These oscillations thus influence the film cooling coverage, and the time-averaged benefit of film cooling is influenced by the oscillation.

Thermal effects on breakthrough curves of pressure swing adsorption for hydrogen purification

Applying pressure swing adsorption (PSA) to recover hydrogen from industrial exhaust gas is an effective way to recycle by-product industrial exhaust gas. In this paper, the PSA process for hydrogen purification is studied using a one-dimensional multicomponent adsorption, heat and mass transfer model which is implemented in Comsol Multiphysics version 3.5a. This model is validated by experimental data on activated carbon (AC) and zeolite 5A bed. Both the breakthrough curve and the bed temperature distribution show good agreement with experiments. This paper considers two hydrogen ternary mixtures (H2/CH4/CO = 60.4/28.1/11.5 vol% in activated carbon bed; H2/CH4/CO = 60/30/10 vol% in zeolite 5A bed) and one hydrogen quaternary mixture (H2/CO2/CH4/CO = 69/26/3/2 vol% in activated carbon bed). The purpose of two ternary mixtures (H2/CH4/CO) that are fed into an activated carbon PCB bed and a zeolite 5A bed respectively is to study the influence of adsorbent on the breakthrough characteristics of the mixture. The quaternary mixture (H2/CO2/CH4/CO) is fed into an activated carbon bed aimed at investigating the effect of mixture composition on adsorption performance. The thermal effects associated with the adsorption process are examined by comparing various thermal boundary conditions: adiabatic, isothermal and general heat transfer boundary conditions. The results suggest that good control of bed temperature is an effective way to restrain thermal effects on breakthrough curves. The operating parameters of feed flow rate and axial mass dispersion coefficient are also examined.

Investigation of Film Cooling Effectiveness Enhancement by Using Corrugated Spiral Holes

Film cooling represents one of the important game-changing technologies that has allowed the achievement of today’s high inlet turbine temperature to get high-efficiency gas turbine engines. Shear Stress Transport (SST) K-? was performed to investigate the flow structures and the development process of vortices in different sections (rectangular, hexagonal and circular) of a stationary gas turbine at blowing ratio equal to (0.5, 1.0, 1.5 and 2.0), while the jet film coolinghole angles equal to (90°, 60°, 45° and 30°). It was observed that higher freestream turbulence improves adiabatic effectiveness and heat transfer coefficient for the corrugation film cooling holes design and specially enhancement at holes angle equal to 30° with twisted angle equal to 360° and rectangular corrugation shape. Results show the effectiveness enhancement range of 20.1%-69.3%of the smooth film cooling hole. Numerical calculation of film cooling effectiveness is validated with reported numerical and experimental results

Infrared thermography to study endwall cooling and heat transfer in turbine stator vane passages using the auxiliary wall method and comparison to numerical simulations

Experiments using the auxiliary wall method and infrared thermography allow to study film cooling and heat transfer in turbomachinery research with high spatial resolution. Using heater foils and pulse width modulation, an aluminium body is heated to constant wall temperature, controlled by thermocouples. The heat flux is then determined across a low conductivity layer of Ethylene-Tetrafluoroethylene (ETFE), whereby 1-D conduction is assumed. Setting the base body to several quasi-steady state wall temperatures allows to deduce adiabatic wall temperatures and heat transfer coefficients. Given a coolant and main flow with different temperature, cooling effectiveness can be calculated, using a superposition approach. Experiments in the linear cascade test rig of the Institute for Gas Turbines and Aerospace Propulsion have been performed to study the effect of hub side coolant injection on the endwall heat transfer of a turbine stator row. The quality of results is examined through extensive data analysis, accompanied by a numerical simulation of the experiment.

Numerical Analysis of the Conjugate Cooling of a 2D Protruding Heater in Laminar Channel Flow

The conjugate forced convection-conduction heat transfer from a 2D protruding block mounted on the lower conductive wall of a parallel plates channel was numerically investigated, using the control volumes method together with the SIMPLE algorithm. A uniform heat generation in the protruding block was transferred either directly by convection to an airflow in the channel or by conduction to the lower wall. The analysis was performed under steady state conditions, for laminar airflow with constant properties in the dynamic and thermal entrance region of the channel. This problem is related to the cooling of electronic components assembled on printed circuit boards (PCB). The board acts as a substrate providing a path for conductive heat spreading through its wall. This heat transfer eventually returns to the airflow by convection at the substrate surface. A fraction of this convective heat transfer occurs upstream of the 2D heated block and preheats the airflow before it reaches the heater surfaces. Due to this effect, it is convenient to describe the direct convection from the heater surfaces to the airflow by the adiabatic heat transfer coefficient. The considered conjugate problem was solved numerically in a domain comprising both the fluid and the solid regions. The heater was considered with a relatively high thermal conductivity and the obtained numerical results showed the effects of parameters such as the channel flow Reynolds number; the heater height in the channel; the substrate thickness; and the substrate to fluid thermal conductivities ratio. The results indicated that within the investigated range of parameters, there is a heat transfer enhancement from the heater at the expense of an increase of its average adiabatic surface temperature.

Uncertainty analysis of adiabatic wall temperature measurements in turbine experiments

During engine service, turbine components are vulnerable to mechanical failures due to large thermal loads. In addition to the heat flux variations during the transients in an engine cycle, high-frequency unsteadiness occurs at every rotor passage with varying amplitudes originating from complex blade row interactions. The heat transfer is therefore time-dependent due to fluctuations in both the convective heat transfer coefficient and in the flow recovery temperature caused by the unsteady work processes within the rotor. It is apparent that the measurement of heat flux alone is not sufficient to elucidate the physical origin of the heat transfer variations, which in turn are the boundary conditions to design the cooling schemes. Hence, the accurate measurement of the adiabatic wall temperature and the adiabatic local convective coefficient becomes essential to ensure a correct thermal assessment of any turbine hardware.This paper illustrates different approaches to estimate the relevant flow parameters that drive the heat transfer based on transient turbine experiments. The work presents a detailed uncertainty analysis associated with the determination of the adiabatic wall temperature and the adiabatic convective heat transfer coefficient based on a multi-test strategy and the use of surface temperature measurements. The present study allows experimental and numerical specialists quantifying the error in the measured or calculated local gas temperature and convective heat transfer coefficient. Conducting such analysis prior to any experimental or numerical campaign will serve to minimize the test uncertainty in adiabatic wall temperature as a function of: turbine operating conditions; wall to gas temperature ratios; heat transfer uncertainty levels; number of experiments.

Numerical investigation of heat transfer effects in small wave rotor

Although a wave rotor is expected to enhance the performance of the ultra-micro gas turbine, the device itself may be affected by downsizing. Apart from the immediate effect of viscosity on flow dynamics when downscaled, the effects of heat transfer on flow field increase at such small scales. To gain an insight into the effects of heat transfer on the internal flow dynamics, numerical investigations were carried out with adiabatic, isothermal and conjugate heat transfer boundary treatments at the wall, and the results compared and discussed in the present study. With the light shed by the discussion of adiabatic and conjugate heat transfer boundary treatments, this work presents investigations of the heat flux distributions, as well as the effects of heat transfer on the internal flow dynamics and the consequent charging and discharging processes for various sizes. When heat transfer is taken into account, states of fluid in the cell before compression process varies, shock waves in compression process are found to be weaker, and changes in the charging and discharging processes are observed. Heat transfer differences between conjugate heat transfer boundary treatment and isothermal boundary treatment are addressed through comparisons of local wall temperature and heat flux. As a result, the difference in discharging temperature of high pressure fluid is noticeable in all sizes investigated, and the rapid increase of differences between results of isothermal and conjugate heat transfer boundary treatment in small size reveals that for certain small sizes (length of cell < 23 mm) the thermal boundary treatment should be taken care of.

Optimal Low Temperature Charging of Lithium-ion Batteries

A lithium-plating side reaction at the lithiated graphite (LiC6) anode leads to poor safety of the lithium-ion battery. Faster charging at normal temperature may lead to a plating side reaction during the end of charging at the anode-separator interface. At lower temperature, the lithium-plating side reaction may become thermodynamically favorable during almost the entire charging period, even at low rates. This paper uses an electrochemical engineering model and dynamic optimization framework to derive charging profiles to minimize lithium plating at low temperatures. Transport parameters for lithium-ion battery are very sensitive at low temperatures. This paper shows the derivation of the optimal charging profile considering strict lower bounds on the plating reaction depending on various thermal insulation conditions (adiabatic, isothermal, and normal heat transfer coefficient) surrounding the battery.

Dynamic Identification of Thermodynamic Parameters for Turbocharger Compressor Models

A novel experimental procedure is presented which allows simultaneous identification of heat and work transfer parameters for turbocharger compressor models. The method introduces a thermally transient condition and uses temperature measurements to extract the adiabatic efficiency and internal convective heat transfer coefficient simultaneously, thus capturing the aerodynamic and thermal performance. The procedure has been implemented both in simulation and experimentally on a typical turbocharger gas stand facility. Under ideal conditions, the new identification predicted adiabatic efficiency to within 1% point1 and heat transfer coefficient to within 1%. A sensitivity study subsequently showed that the method is particularly sensitive to the assumptions of heat transfer distribution pre- and postcompression. If 20% of the internal area of the compressor housing is exposed to the low pressure intake gas, and this is not correctly assumed in the identification process, errors of 7–15% points were observed for compressor efficiency. This distribution in heat transfer also affected the accuracy of heat transfer coefficient which increased to 20%. Thermocouple sensors affect the transient temperature measurements and in order to maintain efficiency errors below 1%, probes with diameter of less than 1.5 mm should be used. Experimentally, the method was shown to reduce the adiabatic efficiency error at 90 krpm and 110 krpm compared to industry-standard approach from 6% to 3%. However at low speeds, where temperature differences during the identification are small, the method showed much larger errors.

Waveforms of Time-Resolved Film-Cooling Parameters on a Leading-Edge Model

It is necessary to understand how film cooling both reduces the adiabatic wall temperature and influences the heat transfer coefficient in order to predict its benefit to a gas turbine hot gas path component. Although a great number of studies have considered steady film-cooling flows, unsteadiness has only recently been considered. Unsteadiness in the freestream flow or the coolant flow can cause fluctuations in both the adiabatic effectiveness and heat transfer coefficient, the dynamics of which have been difficult to measure. In previous studies, only time-averaged effects have been measured. The present study has determined time-resolved and waveforms using a novel inverse heat transfer methodology. Unsteady interactions between and were examined near a coolant hole on the leading-edge region of a circular cylinder simulating the leading edge of a turbine blade. The coolant plume is shown to shift back and forth as the jet’s momentum fluctuates, resulting in an increased spread of coolant coverage but less than ideal performance at any instant in time. The phase behavior of the and waveforms was also examined; in some locations, they fluctuate in phase, and in others, they fluctuate out of phase, again impacting the overall film-cooling coverage from the steady-state value. This is the first time that time-resolved waveforms for and have been determined experimentally.

Computational Fluid Dynamics Evaluations of Unconventional Film Cooling Scaling Parameters on a Simulated Turbine Blade Leading Edge

While it is well understood that certain nondimensional parameters, such as freestream Reynolds number and turbulence intensity, must be matched for proper design of film cooling experiments; uncertainty continues on the ideal method to scale film cooling flow rate. This debate typically surrounds the influence of the coolant to freestream density ratio (DR) and whether mass flux ratio or momentum flux ratio properly accounts for the density effects. Unfortunately, density is not the only fluid property to differ between typical wind tunnel experiments and actual turbine conditions. Temperature differences account for the majority of the property differences; however, attempts to match DR through the use of alternative gases can exacerbate these property differences. A computational study was conducted to determine the influence of other fluid properties besides density, namely, specific heat, thermal conductivity, and dynamic viscosity. Computational fluid dynamics (CFD) simulations were performed by altering traditional film cooling nondimensional parameters as well as others such as the Reynolds number ratio, Prandtl number ratio, and heat capacity ratio (HCR) to evaluate their effects on adiabatic effectiveness and heat transfer coefficient. A cylindrical leading edge with a flat afterbody was used to simulate a turbine blade leading edge region. A single coolant hole was located 21.5 deg from the leading edge, angled 20 deg to the surface and 90 deg from the streamwise direction. Results indicated that thermal properties can play a significant role in understanding and matching results in cooling performance. Density effects certainly dominate; however, variations in conductivity and heat capacity can result in 10% or higher changes in the resulting heat flux to the surface when scaling ambient rig configurations to engine representative conditions.

Experimental Investigation of Taylor and Intermittent Slug-annular/Annular Flow in Microchannels

High-speed visualization of adiabatic flow and heat transfer rate determination for constant wall heat flux conditions were performed to study the flow and heat transfer behavior of non-boiling, gas–liquid two-phase vertical upward flow in a 2.00 mm-diameter tube. Liquids of different volatilities, including water, ethylene glycol, and hexadecane, were employed to investigate the roles of convective heat transfer and evaporation for a wide range of flow conditions encompassing Taylor, slug-annular, and annular flow regimes. The heat transfer rate is found to depend strongly on the flow regime. Significant evaporative cooling was observed for the volatile system at high gas flow rates. A heat transfer enhancement up to fivefold over that for the liquid-only flow was observed in the annular flow regime.

Introducing Some Correlations to Calculate Entropy Generation in Extended Surfaces with Uniform Cross Sectional Area

The optimum length of extended surfaces with uniform cross sectional area has been analyzed numerically, based on the concept of entropy generation minimization. The extended surface studied is a pin fin. The rate of entropy generation is investigated for different boundary conditions. First, some correlations are introduced to calculate this rate, and then a model is offered to find optimum length of the fin for adiabatic and convection heat transfer boundary conditions. The accuracy of the model presented is compared with experimental data. Although Bejan introduced a correlation to calculate optimal Reynolds number and consequently the optimum length of a pin fin, but the results showed the new method has high accuracy compared with the Bejan method. Also, it is found that there is a strong relation between optimum length (based on the entropy generation minimization concept) in one side, and temperature distribution in the other side.

Impact of the Combustor-Turbine Interface Slot Orientation on the Durability of a Nozzle Guide Vane Endwall

The combustor-turbine interface is an essential component in a gas turbine engine as it allows for thermal expansion between the first stage turbine vanes and combustor section. Although not considered as part of the external cooling scheme, leakage flow from the combustor-turbine interface can be utilized as coolant. This paper reports on the effects of orientation of a two-dimensional leakage slot, simulating the combustor-turbine interface, on the net heat flux reduction to a nozzle guide vane endwall. In addition to adiabatic effectiveness and heat transfer measurements, time-resolved, digital particle image velocimetry (TRDPIV) measurements were performed in the vane stagnation plane. Four interface slot orientations of 90 deg, 65 deg, 45 deg, and 30 deg located at 17% axial chord upstream of a first vane in a linear cascade were studied. Results indicate that reducing the slot angle to 45 deg can provide as much as a 137% reduction to the average heat load experienced by the endwall. Velocity measurements indicate the formation of a large leading edge vortex for coolant injected at 90 deg and 65 deg while coolant injected at 45 deg and 30 deg flows along the endwall and washes up the vane surface at the endwall junction.

Analysis of the Unsteady Overtip Casing Heat Transfer in a High Speed Turbine

In modern gas turbine engines, the rotor casing is vulnerable to thermal failures due to large unsteady heat fluxes. The rotor tip flow unsteadiness is induced by the periodic passage of the rotor blades, with an intensity dependent on the tip gap geometry. Hence, the understanding of the physics is of paramount importance to develop appropriate predictive tools and improve the cooling schemes. The present research aims at providing essential information on the flow conditions, which should serve to assess the relative impact of the overtip flow, tip gap magnitude, and work extraction processes on the casing thermal load. This paper presents simultaneous measurements of steady and unsteady heat transfer, pressure and rotor tip clearance in the casing of a transonic turbine stage. The research article was tested in a compression tube facility operating at engine representative conditions (vane Mach number 1.07, vane outlet Reynolds number 1.3 × 106, pressure ratio is 2.92, at 6790 rpm). The rotor blade geometry has a flat tip with a nominal tip clearance of about 0.4% of blade height. The heat transfer, pressure, and tip clearance data were obtained at three circumferential positions around the turbine casing. The heat flux was monitored using a single-layered thin film gauge able to resolve with high fidelity the wall temperature fluctuations. The heat flux sensor was mounted on a probe equipped with a heating device that allows varying the wall temperature. A series of experiments was performed at different heating rates to derive the local adiabatic wall temperature and the adiabatic convective heat transfer coefficient. A high bandwidth capacitive sensor provided the instantaneous value of the single blade tip clearance. A simple zero-dimensional model has been proved effective to predict the local flow temperature while the rotor spins up prior to the test, and estimate the overtip flow temperature during a test.