Heat Transfer Coefficient Distributions Research Articles

The Effect of variation Longitudinal Ribs Height in Spanwise Direction on Flow and Heat Transfer Characteristics in a Rectangular Duct

The present work has investigated numerically and experimentally the effect of a streamwise riblet on the flow and heat transfer characteristics for fully developed turbulent flow in a rectangular duct heated with constant heat flux for Reynolds number based on hydraulic diameter range of (1.5×〖10〗^4-6×〖10〗^4). Numerical simulations have been done by solving thegoverning equations (Continuity, Reynolds. Averaging Navier-stokes and Energy) in turbulent regime with appropriate turbulence model Shear-Stress Transport (k-ω) in three dimensions by using the FLUENT version (12.1.2). The variation of peak to peak height of riblet in spanwise direction ratio with 〖(h〗_s⁄h_L =0.5)was simulated. The flow structure and heat transfer characteristics (the velocity contours, vorticity contours, secondary flow contours, temperature contours, the distribution of local wall shear stress and heat transfer coefficient in spanwise direction at ribs)were presented as results. Also the effects of ribs on the flow and heat transfer characteristic at duct were presented, as the percentage of average skin friction, Stanton number and the heat transfer efficiency relative to a smooth surface. Experiments were conducted for rectangular duct of aspect ratio =10. Experimental results indicated an increase in the thickness of hydraulic and thermal sub-layer and shifted up the velocity profile, reducing the skin friction coefficient and Stanton number by about (6 %) and (23%) respectively. The experimental results gave a good agreement with the numerical simulation and previous works.

Experimental and numerical study on transient heat transfer for a twisted plate with different length in helium gas at various velocities

ABSTRACTThis study is aimed to investigate the transient heat transfer process between the solid surface and the coolant (helium gas) in very high temperature reactor or intermediate heat exchanger. Transient heat transfer from a twisted plate with different length in helium gas was experimentally and theoretically studied. The heat generation rate was increased with an exponential function, Q = Q0exp(t/τ), where t is time and τ is period. Experiment was carried out at various periods ranged from 35 ms to 14 s. Platinum plates were twisted with the same helical pitch of 20 mm, and the effective lengths are 26.8, 67.8 and 106.4 mm (pitch numbers of 1, 3 and 5), respectively. It was clarified that the average heat transfer coefficient approaches quasi-steady-state value when the period τ is larger than about 1 s, and it becomes higher when τ is shorter than about 1 s. The heat transfer coefficient decreases with the increase of plate length. An empirical correlation for forced convection heat transfer for a twisted plate with various lengths was obtained based on the experimental data. Moreover, numerical simulation results were obtained for average surface temperature difference, heat flux and heat transfer coefficient of the twisted plates with different length and showed reasonable agreement with experimental data. Through the numerical simulation, distribution of heat transfer coefficient on heater surface, temperature distribution and velocity distribution were clarified.

Effect of fan-generated swirl on turbulent heat transfer and fluid flow in a pipe

Fan-generated swirl can be characterized as chaotic and strongly mixed. It differs from controlled swirl with respect to those characteristics that tend to increase both the rate of heat transfer and the pressure drop. In the investigation described here, a rotating fan simulation, which takes full account of the rotation of the fan blades and of the true nature of the delivered flow, was implemented with high fidelity. The fan output is delivered directly to the inlet of a round pipe. It is believed that this is the first time that such a realistic approach has been used. The air delivered to the pipe has a temperature different from that of the pipe wall. The swirl component is shown to give rise to a significant enhancement of heat transfer, from a factor of two to 50% between x/D=0 and 20. Similar increases of the wall shear were also found to result from the swirl. As a comparison case, a blower curve for the fan in question was used. Since blower curves omit the swirl component, only a uniform axial flow can be extracted. The swirl effects on heat transfer and shear stress were obtained by comparing the results for the actual rotating-fan-provided flow with those for the blower curve model. In the absence of swirl, the axial distributions of the local heat transfer coefficient and wall shear displayed a local undershoot, thereby supporting limited earlier reporting of such happenings in simpler situations.

Effect of thermal boundary conditions and thermal conductivity on conjugate heat transfer performance in pin fin arrays

The present work experimentally investigates the conjugate heat transfer performance for pin fin arrays. The geometry of pin fin arrays is typical of x/D=y/D=2.5 and H/D=1. The effect of two main factors: material thermal conductivity and thermal boundary conditions, i.e. conjugate and convective boundary conditions, are quantified by comparing the nondimensional temperature and Nusselt number.For conjugate heat transfer, models are constructed with materials with thermal conductivity ranging from 0.23Wm−1K−1 to 16Wm−1K−1. Uniform heat flux is imposed along the external wall of pin fin arrays and highly resolved temperature distributions of internal wall is obtained with steady liquid crystal, meanwhile external wall temperature is measured through thermocouples. For convective heat transfer, model is constructed with low thermal conductivity material to ensure the usage of transient liquid crystal to obtain heat transfer coefficients of the internal wall on the same configuration.Experimental data of temperature and heat transfer coefficient distribution is used as boundary conditions to conduct FEM calculations of pin fin array. Internal and external wall non-dimensional temperature distributions, as well as iso-thermal line distributions of the whole domain, are compared. Results indicate that thermal conductivity can significantly impact the heat transfer capacity, i.e. Nusselt number; meanwhile, the necessity of taking conjugate heat transfer effect is demonstrated by comparison with purely convective results.

Numerical study on film cooling and convective heat transfer characteristics in the cutback region of turbine blade trailing edge

Gas turbine blade trailing edge is easy to burn out under the exposure of high-temperature gas due to its thin shape. The cooling of this area is an important task in gas turbine blade design. The structure design and analysis of trailing edge is critical because of the complexity of geometry, arrangement of cooling channels, design requirement of strength, and the working condition of high heat flux. In the present paper, a 3-D model of the trailing edge cooling channel is constructed and both structures with and without land are numerically investigated at different blowing ratio. The distributions of film cooling effectiveness and convective heat transfer coefficient on cutback and land surface are analyzed, respectively. According to the results, it is obtained that the distributions of film cooling effectiveness and convective heat transfer coefficient both show the symmetrical characteristics as a result of the periodic structure of the trailing edge. The increase of blowing ratio significantly improves the film cooling effectiveness and convective heat transfer coefficient on the cutback surface, which is beneficial to the cooling of trailing edge. It is also found that the land structure is advantageous for enhancing the streamwise film cooling effectiveness of the trailing edge surface while the film cooling effectiveness on the land surface remains at a low level. Convective heat transfer coefficient exhibits a strong dependency with the blowing ratio, which suggests that film cooling effectiveness and convective heat transfer coefficient must be both considered and analyzed in the design of trailing edge cooling structure.

CFD Study on Convective Heat Exchange between Impinging Gas Jets and Solid Surfaces

The paper is concerned with a CFD numerical investigation using the Fluent code of the convective heat exchange between an impinging gas jet at room temperature and a structured solid surface with square-shaped grooves which is maintained at constant temperature of 80°C. Reynolds averaged Navier-Stokes simulations coupled with the energy equation are performed for a planar jet on a 3D domain with periodic boundary conditions.The range of jet Reynolds numbers used for the simulations is in the interval from 2000 to 10000, while the jet standoff distance can take different values, ranging from 2 to 10 times the jet width b=2mm. The detailed distribution of the convective heat transfer coefficient and the mean Nusselt number at the impingement of the air jet on the impacted plate will be inferred. Finally, a comparison between the numerical results, a set of experiments performed and relevant cases from literature is made and conclusions are drawn. One main result of the study is the confirmation of the numerical procedure for the domain used.

The Relative Performance of External Casing Impingement Cooling Arrangements for Thermal Control of Blade Tip Clearance

As a key way of improving jet engine performance, a thermal tip clearance control system provides a robust means of manipulating the closure between the casing and the rotating blade tips, reducing undesirable tip leakage flows. This may be achieved using an impingement cooling scheme on the external casing. Such systems can be optimized to increase the contraction capability for a given casing cooling flow. Typically, this is achieved by changing the cooled area and local casing features, such as the external flanges or the external cooling geometry. This paper reports the effectiveness of a range of impingement cooling arrangements in typical engine casing closure system. The effects of jet-to-jet pitch, number of jets, and inline and staggered alignment of jets on an engine representative casing geometry are assessed through comparison of the convective heat transfer coefficient distributions as well as the thermal closure at the point of the casing liner attachment. The investigation is primarily numerical, however, a baseline case has been validated experimentally in tests using a transient liquid crystal technique. Steady numerical simulations using the realizable k–ε, k–ω SST, and EARSM turbulence models were conducted to understand the variation in the predicted local heat transfer coefficient distribution. A constant mass flow rate was used as a constraint at each engine condition, approximately corresponding to a constant feed pressure when the manifold exit area is constant. Sets of local heat transfer coefficient data generated using a consistent modeling approach were then used to create reduced order distributions of the local cooling. These were used in a thermomechanical model to predict the casing closure at engine representative operating conditions.

Direct contact evaporation heat transfer coefficient and drobble size distribution in a 2D column

This work presents an experimental study of the n-pentane evaporation process in hot water to investigate the behaviour of drobbles and the performance of direct contact heat transfer. The evaporating drobble size distribution (DSD), Sauter mean diameters, interfacial areas of liquid–liquid heat transfer and n-pentane evaporation rate are measured at different n-pentane inlet flowrates and water temperatures. At the bottom of the 2D column, the probability density function (PDF) has a unimodal shape at a water temperature of 45 °C and has a bimodal shape at a relatively high water temperature of 55 °C. The Sauter mean diameter of the drobbles increases with increasing water temperature and height of the test position. The liquid–liquid heat transfer interfacial area decreases upon increasing the testing height and water temperature. The heat transfer coefficient of a single drobble has a large dependence upon the rising velocity.

Investigation of heat transfer in spacer-filled channels by experiments and direct numerical simulations

The analysis of flow fields and heat or mass transfer phenomena is of great importance in the optimum design of spacer-filled channel geometries for a variety of membrane-based processes. In the present work, models of spacer-filled channels often adopted in Membrane Distillation are simultaneously investigated by experiments and Computational Fluid Dynamics (CFD). Experiments rely on a non-intrusive technique, based on the use of Thermochromic Liquid Crystals (TLC) and digital image processing, and provide the local distribution of the convective heat transfer coefficient on a thermally active wall. CFD relies on steady-state (laminar flow) simulations in the lower end of the Reynolds number range investigated and on direct numerical simulations in the upper end of this range. This latter is a region of great practical interest for real applications, in which the flow is chaotic but not fully turbulent, and neither steady-state simulations nor the use of turbulence models would provide satisfactory predictions. Results are reported and discussed for a specific spacer geometry (overlapped orthogonal cylindrical filaments) and different spacer orientations with respect to the main flow. To the authors’ knowledge, this is the first study in which an experimental validation of computational results concerning local heat or mass transfer is performed for spacer-filled channels.

Heat Transfer Investigation on an Internal Cooling System of a Gas Turbine Leading Edge Model

A scaled up test model simulating a realistic leading edge cooling system of a high pressure gas turbine blade was designed with the aim of performing heat transfer measurements in static and rotating conditions.The test model is composed by a trapezoidal supply channel which feeds three large racetrack holes, generating coolant impingement on the internal concave leading edge surface. Four big fins allow to confine the impingement jets impact zones. Air is then extracted through 4 rows of 6 holes each, two of showerhead (SH) and two of film cooling (FC). The test model is installed on a rotating test rig, which allows to reach jet Reynolds numbers (Rej) up to 40000 and rotation numbers (Roj) up to 0.05. The effect of cross-flow in the supply channel is also considered.The heat transfer coefficient (HTC) distribution on the internal concave surface was evaluated by means of a steady state technique, using wide band thermochromic liquid crystals (TLCs) to measure the wall temperature and an electrically heated Inconel sheet to provide a constant heat flux to the investigated surface.This paper reports experimental results obtained in static conditions for Rej 10000 and 30000 and for two cross-flow cases representative of blade tip and hub sections. The effects of different mass flow extraction between pressure and suction side is also investigated by varying the mass flow rate through FC and SH holes.The effect of the coolant extraction holes geometry on the Nusselt number distribution is analyzed by comparing the experimental results, reported as 2D Nusselt number (Nu) maps, with a previous investigation on an analogous test model with similar impingement geometry. A CFD campaign was also carried out on the present test rig, exploiting a previously validated computational model. Both numerical and experimental results reveal that the effects of differentiated mass flow extraction and extraction area modification are secondary with respect to the effects of Rej and crossflow variation.

Experimental Investigation of Heat Transfer Coefficient from the Impingement of a Slot Jet Using Conjugate Gradient Method with Adjoint Equation

The convective heat transfer coefficients resulting from a slot jet on a plane surface have been estimated by using the real measured temperatures in the plate and inverse method. In this study, the inverse method used the conjugate gradient method with an adjoint equation. Distributions of the local heat transfer coefficient on the impingement surface were determined for various Re and H/Dh. It was found the heat transfer coefficients generally tended to decrease with increasing separation distance and to increase with an increase in Reynolds number. This presented method is able to estimate the variation of the local Nusselt number with time.

Numerical Simulation of the Fluid Flow, Heat Transfer, and Solidification During the Twin-Roll Continuous Casting of Steel and Aluminum

The commercialization of aluminum twin-roll casting was realized in the early 1950s, while it is still a dream for engineers to produce steel strip by this process. In the present paper, a two-dimensional mathematical model is employed to study the fluid flow, heat transfer, and solidification during the twin-roll casting for both steel and aluminum. The turbulent flow in the pool is examined using the Lam and Bremhorst low-Reynolds-number turbulence model. In order to facilitate the comparison and analysis, a new transformed coordinate system (r, φ) is established. Characteristics of the momentum boundary layer and the solidification front are described. Reasons of the formation of the wedge-shaped zone near the surface of rotating roll are given. In the transformed coordinate system (r, φ), the effect of the centrifugal force induced by the rotating roll is presented using the velocity component in the r direction and the pressure gradient in the r direction. At last, the evaluation of the solidified shell in the pool is analyzed. The results show that the twin-roll casting is a roll-rotating-driven process. The variation of the thickness of the momentum boundary layer can be divided into three stages and its thickness is very uniform at the last stage. Near the roll surface, there exists a wedge-shaped zone induced by the near-roll-surface shear flow that washes the mushy zone front, which increases the depth of the liquid pool and decreases the length of the rolling region. The rotating roll gives rise to the stirring effect to the pool region and the metal is moving away from the roll surface in the positive radial velocity region, and the effect of the centrifugal force becomes weak in the lower part of the pool. At the solidification front, the non-dimensional effective heat transfer coefficient distribution in steel twin-roll casting is larger than that in aluminum twin-roll casting. Considering that the turbulence level is determined by the flow pattern in the pool region, which demonstrates the importance of the geometry of the feeding system in steel twin-roll casting. The evaluation of the solidified shell in aluminum twin-roll casting is a parabolic growth, while in steel twin-roll casting, the parabolic growth only occurs in the lower part of the pool.

Effect of Slot Injection and Effusion Array on the Liner Heat Transfer Coefficient of a Scaled Lean-Burn Combustor With Representative Swirling Flow

International standards regarding polluting emissions from civil aircraft engines are becoming gradually even more stringent. Nowadays, the most prominent way to meet the target of reducing NOx emissions in modern aero-engine combustors is represented by lean-burn technology. Swirl injectors are usually employed to provide the dominant flame stabilization mechanism coupled to high-efficiency fuel atomization solutions. These systems generate very complex flow structures, such as recirculations, vortex breakdown, and processing vortex core, which affect the distribution and therefore the estimation of heat loads on the gas side of the liner as well as the interaction with the cooling system flows. The main purpose of the present work is to provide detailed measurements of heat transfer coefficient (HTC) on the gas side of a scaled combustor liner highlighting the impact of the cooling flows injected through a slot system and an effusion array. Furthermore, for a deeper understanding of the interaction phenomena between gas and cooling flows, a standard two-dimensional (2D) particle image velocimetry (PIV) technique has been employed to characterize the combustor flow field. The experimental arrangement has been developed within EU project LEMCOTEC and consists of a nonreactive three sectors planar rig installed in an open-loop wind tunnel. Three swirlers, replicating the real geometry of a GE Avio partially evaporated and rapid mixing (PERM) injector technology, are used to achieve representative swirled flow conditions in the test section. The effusion geometry is composed by a staggered array of 1236 circular holes with an inclination of 30 deg, while the slot exit has a constant height of 5 mm. The experimental campaign has been carried out using a thermochromic liquid crystals (TLCs) steady-state technique with a thin Inconel heating foil and imposing several cooling flow conditions in terms of slot coolant consumption and effusion pressure drop. A data reduction procedure has been developed to take into account the nonuniform heat generation and the heat loss across the liner plate. Results in terms of 2D maps and averaged distributions of HTC have been supported by flow field measurements with 2D PIV technique focussed on the corner recirculation region.

Schlieren visualization of water natural convection in a vertical ribbed channel

Schlieren techniques are valuable tools for the qualitative and quantitative visualizations of flows in a wide range of scientific and engineering disciplines. A large number of schlieren systems have been developed and documented in the literature; majority of applications involve flows of gases, typically air. In this work, a schlieren technique is applied to visualize the buoyancy-induced flow inside vertical ribbed channels using water as convective fluid. The test section consists of a vertical plate made of two thin sheets of chrome-plated copper with a foil heater sandwiched between them; the external sides of the plate are roughened with transverse, square-cross-sectioned ribs. Two parallel vertical walls, smooth and unheated, form with the heated ribbed plate two adjacent, identical and asymmetrically heated, vertical channels. Results include flow schlieren visualizations with colour-band filters, reconstructions of the local heat transfer coefficient distributions along the ribbed surfaces and comparisons with past experiments performed using air as working fluid.

Heat Transfer Augmentation Due to Coolant Extraction on the Cold Side of Active Clearance Control Manifolds

Jet array is an arrangement typically used to cool several gas turbine parts. Some examples of such applications can be found in the impingement cooled region of gas turbine airfoils or in the turbine blade tip clearances control of large aero-engines. In the open literature, several contributions focus on the impingement jets formation and deal with the heat transfer phenomena that take place on the impingement target surface. However, deficiencies of general studies emerge when the internal convective cooling of the impinging system feeding channels is concerned. In this work, an aerothermal analysis of jet arrays for active clearance control (ACC) was performed; the aim was the definition of a correlation for the internal (i.e., within the feeding channel) convective heat transfer coefficient augmentation due to the coolant extraction operated by the bleeding holes. The data were taken from a set of computational fluid-dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) simulations, in which the behavior of the cooling system was investigated over a wide range of fluid-dynamics conditions. More in detail, several different holes arrangements were investigated with the aim of evaluating the influence of the hole spacing on the heat transfer coefficient distribution. Tests were conducted by varying the feeding channel Reynolds number in a wide range of real engine operative conditions. An in depth analysis of the numerical data set has underlined the opportunity of an efficient reduction through the local suction ratio (SR) of hole and feeding pipe, local Reynolds number, and manifold porosity: the dependence of the heat transfer coefficient enhancement factor (EF) from these parameter is roughly exponential.

LES of rotational effects on film cooling effectiveness and heat transfer coefficient in a gas turbine blade with one row of air film injection

The effects of rotation on film cooling effectiveness and heat transfer coefficient distributions on the suction and pressure surfaces of a gas turbine blade were numerically simulated using large eddy simulation method. The flow is in the low Mach number (incompressible) regime. The suction (convex) side has simple angled cylindrical film-cooling holes; the pressure (concave) side has compound angled cylindrical film-cooling holes. The blowing ratio (BR) is 0.5 on both the suction and pressure sides and the inlet Reynolds number based on the axial chord is 3 × 105. The effects of turbine rotating conditions on the film cooling effectiveness and heat transfer coefficient distributions were investigated at four rotating speeds of 0 rpm, 125 rpm, 250 rpm, and 500 rpm. Air was injected through one row of film holes each on the pressure and suction surfaces. The commercial code STAR-CCM+ was used in the prediction. The results indicate that film cooling effectiveness increases with an increase in the rotating speed. Higher turbine rotating speeds show more local film cooling effectiveness spread on pressure surface. The film cooling effectiveness and Nusselt number distributions were presented along with the discussions on the influences of rotating speed. Results showed that the rotation promotes an earlier boundary layer transition and increases the transition length on the suction surface.

Heat transfer and pressure drop measurements in a square cross-section converging channel with V and W rib turbulators

Ribs on the opposite walls of internal cooling passages of gas turbine blades are often used for heat transfer enhancement. These passages can be straight, converging or diverging. In the present study, experimental data for local heat transfer coefficients are presented for a converging channel with rib roughening elements with the cross-section being maintained square from inlet to exit. The local heat transfer coefficient distribution shows the same qualitative behavior observed for the straight channel. The overall averaged heat transfer coefficient along with the measured pressure drop across the test channel is used to calculate thermal performance based on constant pumping power and constant heat transfer area criterion.Data are presented for straight ribs normal to the flow, V ribs with included apex angle of 45° and with apex pointing in the upstream and downstream directions, W ribs with all included angles of 45° and with central apex pointing in the upstream and downstream directions. The inlet Reynolds number (Rein) was varied from 5000 to 35,000. The rib height (e) was maintained constant throughout the length of the channel. The rib height to mean duct hydraulic diameter ratio (e/Dh,m) was kept constant at 0.08. Data for three pitch to height ratios (P/e) equal to 6, 10 and 17.5 are reported for straight and V ribs. The optimum P/e ratio based on constant pumping power thermal performance criterion was observed to be equal to 10 and this was the only configuration studied for W ribs. The difference in thermal performance between the V and W ribs was within uncertainty limits. However, the local variation in heat transfer coefficient in the cross stream direction on the ribbed wall was noticed to be very high for the V ribs compared to that for the W ribs making the W ribs a better choice as enhancement device.

Realistic solar heating in urban areas: Air exchange and street-canyon ventilation

Numerical fluid flow and heat transfer simulations of a three-dimensional idealized urban environment are performed to investigate the effect of realistic non-uniform thermal forcing that is caused by solar insolation and inter-building shadowing. Simulations at different times of day are performed using Large Eddy Simulation (LES) and mean flow and turbulence statistics are investigated as determinants for urban canyon ventilation. The differential surface heating of the building canyon is parameterized using sets of horizontal and vertical Richardson numbers indicating atmospheric instability and solar tilt with respect to the wind direction, respectively. Roof heating, in combination with building walls and ground heating, is shown to be important in the strength and location of the canyon vortex. For example, in case of weak vertical stratification (Riv) and high horizontal temperature gradient opposing the wind direction above the canyon (Rih), the combination of roof heating and ground heating decreases the strength of the canyon vortex. The distribution of local convective heat transfer coefficients (CHTC) on building facets are analyzed. Throughout the day, the windward walls exhibit larger CHTCs and leeward heating enhances CHTC from the roof and windward walls. Additionally, the wall heating in the cross-stream canyon (north-south in our case) enhances the air exchange rate from the canyon, especially when the leeward wall is heated.

Effect of flue gas recirculation on heat transfer in a supercritical circulating fluidized bed combustor

Abstract This paper focuses on assessment of the effect of flue gas recirculation (FGR) on heat transfer behavior in 1296t/h supercritical coal-fired circulating fluidized bed (CFB) combustor. The performance test in supercritical CFB combustor with capacity 966 MWth was performed with the low level of flue gas recirculation rate 6.9% into furnace chamber, for 80% unit load at the bed pressure of 7.7 kPa and the ratio of secondary air to the primary air SA/PA = 0.33. Heat transfer behavior in a supercritical CFB furnace between the active heat transfer surfaces (membrane wall and superheater) and bed material has been analyzed for Geldart B particle with Sauter mean diameters of 0.219 and 0.246 mm. Bed material used in the heat transfer experiments had particle density of 2700 kg/m3. A mechanistic heat transfer model based on cluster renewal approach was used in this work. A heat transfer analysis of CFB combustion system with detailed consideration of bed-to-wall heat transfer coefficient distributions along furnace height is investigated. Heat transfer data for FGR test were compared with the data obtained for representative conditions without recycled flue gases back to the furnace through star-up burners.

Impact of ns-DBD plasma actuation on the boundary layer transition using convective heat transfer measurements

This paper demonstrates that the impact of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators on the structure of the boundary layer can be investigated using quantitative convective heat transfer measurements. For the experiments, the flow over a flat plate with a C4 leading edge thickness distribution was examined at low speed incompressible flow (6.6–11.5 m s−1). An ns-DBD plasma actuator was mounted 5 mm downstream of the leading edge and several experiments were conducted giving particular emphasis on the effect of actuation frequency and the freestream velocity. Local heat transfer distributions were measured using the transient liquid crystal technique with and without plasma activated. As a result, any effect of plasma on the structure of the boundary layer is interpreted by local heat transfer coefficient distributions which are compared with laminar and turbulent boundary layer correlations. The heat transfer results, which are also confirmed by hot-wire measurements, show the considerable effect of the actuation frequency on the location of the transition point elucidating that liquid crystal thermography is a promising method for investigating plasma-flow interactions very close to the wall. Additionally, the hot-wire measurements indicate possible velocity oscillations in the near wall flow due to plasma activation.