Increase In Surface Heat Flux Research Articles

Improvements in airflow characteristics and effect on the NOSE score after septoturbinoplasty: A computational fluid dynamics analysis.

Septoturbinoplasty is a surgical procedure that can improve nasal congestion symptoms in patients with nasal septal deviation and inferior turbinate hypertrophy. However, it is unclear which physical domains of nasal airflow after septoturbinoplasty are related to symptomatic improvement. This work employs computational fluid dynamics modeling to identify the physical variables and domains associated with symptomatic improvement. Sixteen numerical models were generated using eight patients' pre- and postoperative computed tomography scans. Changes in unilateral nasal resistance, surface heat flux, relative humidity, and air temperature and their correlations with improvement in the Nasal Obstruction Symptom Evaluation (NOSE) score were analyzed. The NOSE score significantly improved after septoturbinoplasty, from 14.4 ± 3.6 to 4.0 ± 4.2 (p < 0.001). The surgery not only increased the airflow partition on the more obstructed side (MOS) from 31.6 ± 9.6 to 41.9 ± 4.7% (p = 0.043), but also reduced the unilateral nasal resistance in the MOS from 0.200 ± 0.095 to 0.066 ± 0.055 Pa/(mL·s) (p = 0.004). Improvement in the NOSE score correlated significantly with the reduction in unilateral nasal resistance in the preoperative MOS (r = 0.81). Also, improvement in the NOSE score correlated better with the increase in surface heat flux in the preoperative MOS region from the nasal valve to the choanae (r = 0.87) than in the vestibule area (r = 0.63). Therefore, unilateral nasal resistance and mucous cooling in the preoperative MOS can explain the perceived improvement in symptoms after septoturbinoplasty. Moreover, the physical domain between the nasal valve and the choanae might be more relevant to patient-reported patency than the vestibule area.

Experimental investigation of heat transfer for diesel spray impingement on a high temperature wall

In this paper, the heat transfer characteristics of spray-wall impingement on a high temperature wall were studied by using a transient thermocouple and a one-dimensional finite-difference conduction model to obtain variations of wall temperature and heat flux. Results showed that increasing the injection pressure and decreasing the ambient temperature both caused an increase in surface heat flux and heat transfer coefficient. However, with the increase of the initial surface temperature from 200 to 600 °C, the surface heat flux and heat transfer coefficient first increased and then decreased, and reached the maximum at about 520 °C and 390 °C respectively, which was due to the change of heat transfer regime on the wall. The contribution of experimental factors descended in the order of initial surface temperature, injection pressure and ambient temperature. The dimensionless surface heat fluxes in terms of Biot and Fourier numbers were highly similar and a dimensionless correlation was developed to quantify this heat transfer behavior, which showed that the ratio of the thermal resistance of the high temperature wall to the thermal resistance of convection heat transfer on the wall surface changed almost linearly during the process of spray-wall impingement.

A Study of One Local‐Scale Convective Precipitation Event Over Central Tibetan Plateau With Large Eddy Simulations

AbstractThe Weather Research and Forecasting (WRF) model with Large Eddy Simulation was employed to investigate a local‐scale convective precipitation event at Naqu (altitude of 4.5 km) over the central Tibetan Plateau (TP) on 24 July 2014. The WRF‐LES simulations with horizontal grid spacing of 200 m (CTRL) basically reproduced the rainfall pattern and evolution process compared to the observations. Five sensitivity experiments including removal the hills in east of Naqu (TER), decrease or increase of surface sensible heat flux (HFX0.67, HFX1.5), and surface latent heat flux (QFX0.67, QFX1.5) were implemented, respectively. Results show that the increase of surface heat flux and the eastern hills both enhanced the convection, which was triggered by the low‐level wind convergence. The QFX0.67 simulated the weakest precipitation among six runs, and the QFX1.5 simulated the earliest and strongest convection due to the enhanced atmospheric instability. It is found that the HFX0.67 caused the earlier convection initiation than CTRL and HFX1.5 owing to the lowest lifting condensation level and wettest low troposphere in it. Over the flat terrain in west of Naqu, weak convection was mostly affected by the surface heat flux. The product of 4.8 km wind convergence and difference between 7 km and 4.8 km equivalent potential temperature (&gt;2 × 10−2 K/s) can partly indicate the development of low‐layer turbulence and the subsequent convection in free atmosphere. In addition, water vapor condensation (extending to 10 km) and raindrop evaporation (below 6 km) were two key phase‐change microphysical processes during the convective period even the quite low 0°C layer (∼6.5 km) over TP.

Global heat balance and heat uptake in potential temperature coordinates

The representation of ocean heat uptake in Simple Climate Models used for policy advice on climate change mitigation strategies is often based on variants of the one-dimensional Vertical Advection/Diffusion equation (VAD) for some averaged form of potential temperature. In such models, the effective advection and turbulent diffusion are usually tuned to emulate the behaviour of a given target climate model. However, because the statistical nature of such a “behavioural” calibration usually obscures the exact dependence of the effective diffusion and advection on the actual physical processes responsible for ocean heat uptake, it is difficult to understand its limitations and how to go about improving VADs. This paper proposes a physical calibration of the VAD that aims to provide explicit traceability of effective diffusion and advection to the processes responsible for ocean heat uptake. This construction relies on the coarse-graining of the full three-dimensional advection diffusion for potential temperature using potential temperature coordinates. The main advantage of this formulation is that the temporal evolution of the reference temperature profile is entirely due to the competition between effective diffusivity that is always positive definite, and the water mass transformation taking place at the surface, as in classical water mass analyses literature. These quantities are evaluated in numerical simulations of present day climate and global warming experiments. In this framework, the heat uptake in the global warming experiment is attributed to the increase of surface heat flux at low latitudes, its decrease at high latitudes and to the redistribution of heat toward cold temperatures made by diffusive flux.

Evolution and heat transfer after droplet impact on heated liquid film with vapor bubbles inside

To better understand the droplet impact on the liquid film with vapor bubbles in spray cooling, a two-dimensional symmetric numerical model is set up using the Coupled Level Set and Volume of Fluid method (CLSVOF). Three simulative cases are taken, considering the effects of film thickness and the presence of vapor bubbles or not. The main purposes of this paper are to investigate the evolution of vapor bubbles during droplet impact and to identify the effect of vapor bubbles on convection heat transfer. The results indicate that vapor bubbles will detach from the wall and break up at the surface of the liquid film during droplet impact, for a thinner film, later a “sawtooth” liquid film appears at the non-impact region. However, for a thicker film, no bubbles rupture and the detached bubbles will flow inside the liquid film and then some will merge into larger bubbles. In the presence of vapor bubbles, the crater radius is larger for a thicker liquid film. The presence of vapor bubbles will facilitate the subcooled droplet to spread to the heated wall, leading to a substantial increase in surface heat flux.

Experimental study on convective boiling flow and heat transfer in a microgap enhanced with a staggered arrangement of nucleated micro-pin-fins

The boiling flow and heat transfer of FC-72 in a microgap channel are experimentally addressed. The heated surface of the microgap with a test area of 10 mm × 10 mm was enhanced using a staggered array of 613 columnar micro pin fins. Inside the micro pin-fins are nucleated with a 60 μm pore and opening of 45 μm. The test surface of the channel is a square of 10 mm, and the height of the channel is 100 μm. The saturation working temperatures of FC-72 is 50 °C, the surface heat flux was up to 60 kW/m2, and the mass flux was selected in the range of 94–275 kg/m2 s. The on nucleated boiling temperatures (ONB), convective heat transfer coefficient, outlet dryness, and pressure drop are measured and reported. The boiling behavior of FC-72 is also recorded using an image capturing device and reported. A comparison between the superheat temperature of nucleated pin fins and plane pin fins (with no nucleation) shows that nucleation of pin fins reduces the superheat temperature. The outcomes reveal that for a high mass flux, 196–275 kg/m2 s, the superheat temperature is high, and the degree of superheat increases as the surface heat flux increases. When the surface heat flux is high (q″ > 30 kW/m2), the convective heat transfer is almost independent of mass flux. At the initiation flow boiling, a notable jump in the pressure drops can be seen. After that, the pressure drop increases gradually by the increase in surface heat flux. The boiling images reveal that the boiling flow initiated inside the nucleation pores of micro pin-fins.

Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

An ensemble of ocean reanalyses for 1815–2013 with sparse observational input

AbstractThis paper describes a new eight‐member ensemble of ocean reanalyses spanning nearly 200 years from 1815 to 2013 generated using the Simple Ocean Data Assimilation system with sparse observational input (SODAsi) to explore long‐term changes in the oceans. The eight ensemble members assimilate surface temperature observations and use surface boundary conditions from an atmospheric reanalysis that is loosely coupled to the ocean reanalysis. Both surface and subsurface quantities, such as dynamic height and heat content, show a broad spectrum of variability. Surface temperature trends from 1815 to 2013 are positive in most regions, with some important exceptions; the central Tropical Pacific, around Antarctica, and in the Gulf Stream and Kuroshio extension regions all show cooling trends. A near‐global average shows warming of about 0.8°C over the full period, with most of the warming occurring after 1920. There is pronounced multidecadal variability in both the midlatitude and tropical oceans. In the North Atlantic Ocean, temperature variability is highly correlated with the meridional overturning stream function, with the largest correlation occurring when the stream function is advanced by 9 years. Trends of upper ocean heat content and dynamic height from the 1950s onward compare well with previously published values. Globally averaged heat content of the upper 700 m shows a nearly linear rise after the 1920s, requiring a net downward surface heat flux increase of 0.47 W m−2 into the ocean. This is close to published estimates of the increased flux required to explain the heat content increase from 1971 to 2010.

Transient heat transfer behavior of water spray evaporative cooling on a stainless steel cylinder with structured surface for safety design application in high temperature scenario

High heat transfer performance of spray cooling on structured surface might be an additional measure to increase the safety of an installation against any threat caused by rapid increase in the temperature. The purpose of present experimental study is to explore heat transfer performance of structured surface under different spray conditions and surface temperatures. Two cylindrical stainless steel samples were used, one with pyramid pins structured surface and other with smooth surface. Surface heat flux of 3.60, 3.46, 3.93 and 4.91 MW/m2 are estimated for sample initial average temperature of 600, 700, 800 and 900 °C, respectively for an inlet pressure of 1.0 MPa. A maximum cooling rate of 507 °C/s was estimated for an inlet pressure of 0.7 MPa at 900 °C for structured surface while for smooth surface maximum cooling rate of 356 °C/s was attained at 1.0 MPa for 700 °C. Structured surface performed better to exchange heat during spray cooling at initial sample temperature of 900 °C with a relative increase in surface heat flux by factor of 1.9, 1.56, 1.66 and 1.74 relative to smooth surface, for inlet pressure of 0.4, 0.7, 1.0 and 1.3 MPa, respectively. For smooth surface, a decreasing trend in estimated heat flux is observed, when initial sample temperature was increased from 600 to 900 °C. Temperature-based function specification method was utilized to estimate surface heat flux and surface temperature. Limited published work is available about the application of structured surface spray cooling techniques for safety of stainless steel structures at very high temperature scenario such as nuclear safety vessel and liquid natural gas storage tanks.

Dramatic effect of elasticity on thermal softening and strain localization during lithospheric shortening

Abstract We present two-dimensional numerical simulations for shortening a viscoelastoplastic lithosphere to quantify the impact of elasticity on strain localization due to thermal softening. The model conserves energy and mechanical work is converted into heat or stored as elastic strain energy. For a shear modulus G = 1010 Pa, a prominent lithospheric shear zone forms and elastic energy release increases the localization intensity (strain rate amplification). For G = 5 × 1010 Pa shear zones still form but deformation is less localized. For G = 1012 Pa, the lithosphere behaves effectively viscoplastic and no shear zones form during homogeneous thickening. Maximal shearing-related increase of surface heat flux is 15–25 mW m−2 and of temperature at lower crustal depth is ∼150 °C, whereby these peak values are transient (0.1–1 My). Elasticity and related energy release can significantly contribute to strain localization and plate-like behaviour of the lithosphere required for plate tectonics.

Couette and Poiseuille flows in a low viscosity asthenosphere: Effects of internal heating rate, Rayleigh number, and plate representation

Mantle convection models with a low viscosity asthenosphere and high viscosity surface plates have been shown to produce very large aspect ratio convection cells like those inferred to exist in Earth’s mantle and to exhibit two asthenospheric flow regimes. When the surface plate is highly mobile, the plate velocity exceeds the flow velocities in the asthenosphere and the plate drives a Couette-type flow in the asthenospheric channel. For sluggish plates, the flow velocities in the asthenosphere exceed the plate velocity and the asthenospheric flow is more Poiseuille-like. It has been shown that under certain circumstances, flows become increasingly Couette-like as the aspect ratio of the plate is increased in numerical simulations. These models also show an increase in the average surface heat flux with aspect ratio which is counterintuitive, as one would expect that large aspect ratio models would result in older and colder oceanic lithosphere. Previous investigations have used single internal heating rates and Rayleigh numbers and a plate formulation that did not preclude significant deformation within the plate. In this paper, we investigate the conditions necessary for Couette and Poiseuille asthenospheric flows and for surface heat flux to increase with plate aspect ratio by varying the internal heating rate, the Rayleigh number and the representation of surface plates in 2D mantle convection models Plates are represented as a high viscosity layer with (1) a free-slip top surface boundary condition and (2) a force-balance boundary condition that imposes a constant surface velocity within the plate. We find that for models with a free-slip surface boundary condition, the internal heating rate and Rayleigh number do not strongly affect the dominance of Couette or Poiseuille flows in the asthenosphere but the increase in surface heat flux with model aspect ratio in the Poiseuille asthenospheric flow regime increases with internal heating rate. For models using the force-balance representation of surface plates, the flow regime in the asthenosphere is found to be almost independent of the plate aspect ratio and spatially averaged surface heat flux decreases with increasing aspect ratio. The dependence of heat flux on plate aspect ratio is an important consideration in Earth’s thermal history while the preponderance of Poiseuille or Couette flows in the asthenosphere may be important for understanding the mobility of surface plates and asthenospheric seismic anisotropy.

Late Pleistocene–Holocene ground surface heat flux changes reconstructed from borehole temperature data (the Urals, Russia)

Abstract. We use geothermal reconstruction of the ground surface temperature (GST) history early obtained in the Middle Urals to determine the surface heat flux (SHF) history over the past 35 kyr. A new algorithm of GST–SHF transformation was applied to solve this problem. The timescale of geothermal reconstructions has been corrected by comparing the estimated heat flux and annual insolation at the latitude of 60° N. The consistency of SHF and insolation changes on the interval 35–6 kyr BP with the linear correlation coefficient R = 0.99 points to orbital factors as the main cause of climatic changes during the Pleistocene–Holocene transition. The amplitude of SHF variations is about 1.3% of the insolation change amplitude. The increase of carbon dioxide concentrations lagged by 2–3 kyr from the SHF increase and occurred synchronously with GST changes.

Secondary reactions of turbulent reacting flows over a film-cooled surface

Demand for greater engine efficiency and thrust-to-weight ratio has driven the production of aircraft engines with higher core temperatures and pressures. Such engines operate at higher fuel–air ratios, resulting in the potential for significant heat release and chemical reactions on a film-cooled surface. Currently, there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. In this paper, the chemically reactive turbulent film cooling over a surface for three cases of an inclined single and two parallel and compound-angle coolant holes is investigated by a Reynolds-averaged Navier–Stokes approach with the shear-stress transport (SST) turbulence model using OpenFoam. To take into account the secondary combustion resulting from the unburned fuels in the crossflow, a two-step reaction scheme was used for the combustion of propane. The relative increase in surface heat flux due to near wall reactions was investigated for film cooling with N2 and air injections. An eddy dissipation concept fast chemistry approach was used to account for the turbulence–chemistry interaction. Results demonstrate that reactions in the turbine cooling film can result in increased heat transferred to the surface. Failure to design for this effect could result in augmented heat transfer caused by the cooling scheme, and turbine life could be degraded substantially. The analysis suggests that high fuel–air ratio designs may have to consider changes to cooling strategies to accommodate secondary combustion.

Experimental Investigation of Effect of Different Types of Surfactants and Jet Height on Cooling of a Hot Steel Plate

Heat transfer studies of a hot AISI 304 stainless steel plate by water jet impingement with different concentrations of three different types of surfactants have been investigated. The study involves a square plate of 100 mm × 100 mm surface area and 6 mm thickness with three subsurface thermocouples positioned at various locations inside the plate. The influence of jet height has been studied by varying the distance between the nozzle and plate from 200 mm to 600 mm. The results show that the heat transfer rate is found to increase with the jet height up to 400 mm and thereafter decreases due to capillary instability of liquid jet. Based on the maximum surface heat flux obtained for a particular nozzle height of 400 mm and an initial surface temperature of 900 °C, further experiments have been carried out with different types of surfactants. The types of surfactants used in the experimental study are anionic surfactant (sodium dodecyl sulphate, SDS), cationic surfactant (cetyltrimethylammonium bromide, CTAB) and nonionic surfactant (Polyoxyethylene 20 sorbitan monolaurate, Tween 20). During cooling, the transient temperature data measured by thermocouples have been analyzed by inverse heat conduction procedure to calculate surface heat flux and surface temperatures. The increase in surface heat flux has been observed with increasing concentration of surfactants and it has been found to be limited to a particular concentration of surfactant after which further increase in concentration leads to decrease in heat flux. Use of surfactant added water minimizes the surface tension and promotes better spreadability of coolant on the test specimen by reducing the solid–liquid contact angle. The maximum heat transfer rate has been found by using nonionic surfactant additive which can primarily be attributed to its lesser foam formability nature.

Influence of Ultrafast Cooling on Microstructure and Mechanical Properties of Steel

The objective of this study is to find the effect of ultrafast cooling on microstructure and mechanical properties of steel with different carbon composition. Steel samples of 6 mm thick AISI 1008, AISI 1040, AISI 1075, and AISI 1090 have been used in the ultrafast cooling experiments to study the effect of carbon percentage on the cooling rate and mechanical properties. Air-atomized spray cooling system has been deployed with a fixed air flow rate and varying water flow rates. Experimental results have shown that, as water flow rate is increased, the cooling rate and surface heat flux increase for all grades of steel. It has been observed that the steel with higher carbon content requires higher water flow rates to reach an ultra fast cooling rate of more than 140°C s−1. After cooling experiments the microstructure, tensile strength and hardness of the samples have been examined and it is found that increase in carbon content affects the cooling rate and also consequently the microstructure. The tensile strength and the hardness of the final product after the ultrafast cooling process conform the advanced high strength steel (AHSS) grade with high martensite phase composition of 65% to 95%.

Subtropical air-sea interaction and development of central Pacific El Niño

The standard deviation of the central Pacific sea surface temperature anomaly (SSTA) during the period from October to February shows that the central Pacific SSTA variation is primarily due to the occurrence of the Central Pacific El Nino (CP-El Nino) and has a connection with the subtropical air-sea interaction in the northeastern Pacific. After removing the influence of the Eastern Pacific El Nino, an S-EOF analysis is conducted and the leading mode shows a clear seasonal SSTA evolving from the subtropical northeastern Pacific to the tropical central Pacific with a quasi-biennial period. The initial subtropical SSTA is generated by the wind speed decrease and surface heat flux increase due to a north Pacific anomalous cyclone. Such subtropical SSTA can further influence the establishment of the SSTA in the tropical central Pacific via the wind-evaporation-SST (WES) feedback. After established, the central equatorial Pacific SSTA can be strengthened by the zonal advective feedback and thermocline feedback, and develop into CP-El Nino. However, as the thermocline feedback increases the SSTA cooling after the mature phase, the heat flux loss and the reversed zonal advective feedback can cause the phase transition of CP-El Nino. Along with the wind stress variability, the recharge (discharge) process occurs in the central (eastern) equatorial Pacific and such a process causes the phase consistency between the thermocline depth and SST anomalies, which presents a contrast to the original recharge/discharge theory.

Effects of nozzle configuration on a shell-and-tube spray evaporator with liquid catcher

Using R-141b as a coolant, this study investigates the efficiency of impact spray cooling heat transfer in a heat exchanger system comprising a horizontal low-finned tube bundle with a triangular-pitch and a fin count of 1000 fins/m. Each copper tube has a base diameter of 19.1 mm (3/4 in.) and is heated by an electrical resistor. The average heat transfer coefficients are calculated by measuring the mean tube surface temperature and the vapor temperature. Six different nozzle configurations are tested to investigate the effect of the nozzle configuration on the spray evaporation heat transfer performance. It is found that the shell-side heat transfer performance depends significantly on the nozzle orifice diameter, the heat flux rate, and the spray mass flow rate, but is influenced only slightly by the height of the nozzle above the upper row of tubes in the bundle. The dry-out phenomenon is observed as the surface heat flux increases. Under these conditions, the spray cooling heat transfer coefficients are less than those obtained in pool boiling. To overcome the dry-out problem, this study fits liquid catchers beneath each tube. The experimental results show that the heat transfer performance for spray cooling with liquid catchers is superior to that of pool boiling under both low and high heat fluxes. Moreover, the influence of the nozzle configuration on the heat transfer performance is significantly reduced when liquid catchers are attached.

Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors: Near Wall Reaction Effects on Film-Cooled Backward-Facing Step Heat Transfer

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions to occur downstream of the combustor is increased. This may take place when partially reacted species leave the combustor and encounter film-cooled surfaces. One common feature on turbine endwalls is a step between various engine components and seals. Such step features produce recirculating flows which when in the vicinity of film-cooled surfaces may lead to particularly severe reaction zones due to long fluid residence times. The objective of this paper is to study and quantify the surface heat transfer implications of such reacting regions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm) flows over a film-cooled backward-facing step. The test article contained two sets of 35 deg film cooling holes located downstream of a step. The film-cooling holes could be supplied with different gases, one side using air and the other nitrogen allowing for simultaneous testing of reacting and inert cooling gases. A mixture of ethylene and argon provided a fuel-rich free stream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5–2.0), and Damköhler numbers (ratio of characteristic flow time to chemical time) from near zero to 30. The experimental results show that for conditions relevant for future engine technology, adiabatic flame temperatures can be approached along the wall downstream of the step leading to potentially significant increases in surface heat flux. A computational study was also performed to investigate the effects of cooling-jet blowing ratio on chemical reactions behind the film-cooled step. The blowing ratio was found to be an important parameter governing the flow structure behind the backward-facing step, and controlling the characteristics of chemical-reactions by altering the local equivalence ratio.

Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors—Part II: Near-Wall Reaction Effects on Film-Cooled Heat Transfer

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions on a film-cooled surface is enhanced. Currently, there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm) flows over a film-cooled flat plate. The test plate contained two sets of 35 deg film cooling holes that could be supplied with different gases, one side using air and the other nitrogen. A mixture of ethylene and argon provided a fuel rich freestream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5–2.0), and Damko¨hler numbers (ratio of flow to chemical time scales) from near zero to 30. For high Damko¨hler numbers, reactions had sufficient time to occur and increased the surface heat flux by 30 percent over the inert cooling side. When these results are appropriately scaled, it is shown that in some situations of interest for gas turbine engine environments significant increases in surface heat flux can be produced due to chemical reactions in the film-cooling layer. It is also shown that the non-dimensional parameters Damko¨hler number (Da), blowing ratio (B), heat release potential (H*), and scaled heat flux Qs are the appropriate quantities to predict the augmentation in surface heat flux that arises due to secondary reactions.

CO 2 laser-induced pulsating regression behavior of GAP at sub-atmospheric pressures

Linear pyrolysis of glycidyl azide polymer (GAP) by CO 2 laser heating was studied experimentally in argon, nitrogen or oxygen at subatmospheric pressures with laser heat fluxes of 1 to 20 W/cm 2. Surface temperature variations were measured by fine chromel-alumel thermocouples pressed onto the sample surface, and pulsating regression caused by an abrupt exothermic decomposition was observed. The behavior is very similar to the chuffing phenomenon. The momentary exothermic decomposition reaction was accompanied by the evolution of N 2 resulting mainly from breaking of the azide bond. The surface temperature in one cycle is characterized by three critical temperatures: liquefying (T l), abrupt thermal decomposition threshold (T ad), and maximum surface (T ms) temperatures. Maximum surface temperatures are 700 to 800 K, which are equivalent to the surface temperature observed during self-sustained combustion controlled chiefly by exothermic reaction at the decomposition surface. As the surface heat flux increases, the period of the pulsating regression decreases, being only slightly dependent on the ambient gas. If the period becomes shorter than the characteristic time of GAP, then the pulsating regression will transition into steady regression assisted by laser irradiation. SEM photographs of the quenched surfaces reveal the presence of a liquid layer with a thickness of 7 to 8 μm, about 1/30 of the momentary regression distance, at the GAP surface just before the abrupt thermal decomposition. Spatial temperature profiles at various critical times were established by using two thermocouples embedded inside the GAP sample. Phenomenological modeling is attempted to explain the pulsating phenomena for GAP pyrolysis. From the phenomenological modeling, a subsurface layer with a relatively small temperature gradient is thought to exist at the GAP surface. In view of SEM photographs of quenched surfaces, the surface layer of the whole momentary regression distance Δl is thought to change into a liquid layer as soon as the abrupt thermal decomposition starts. One of the causes of nonself-sustained combustion of GAP in inert gases of a few atmospheres would be pulsating decomposition. At 66.7 kPa of oxygen gas the pulsating decomposition very often transitions into sustained combustion.