Heat Flux Measurements Research Articles

Novel heat flux controlled surface cooling for hypersonic flight

This paper presents a new method in theory and experiment to adjust the transpiration cooling based on the actual measured heat flux. This is particularly useful in extreme heating environments, e.g. atmospheric entry flight or combustion chamber applications. In such environments, usually cooling is set constant based on the vehicle design, yet a mass efficient and performant cooling is sought after. We present a method with real-time surface heat flux determination of the transpiration cooled wall and an automatic adjustment of the cooling. The heat flux is determined based on a system identification process. The heat flux measurement itself is derived non-intrusively from the measurement of pressure inside the plenum, i.e. the region between mass flow controller and porous wall. The particular advantage of this system is that the heat shield material is not weakened by any sensor system and its performance is optimized with respect to cooling needed at a certain heating level. Another new feature of the pressure heat flux transformation is the attenuation of a destabilizing positive feedback loop, where the transpiration cooling controller’s output (i.e. mass flow rate) strongly influences its input (i.e. plenum pressure). We describe the identification of the model parameters for the heat flux determination, which are found and verified by a calibration approach. The controlled cooling was demonstrated in a hot air plasma flow with a reference heat flux of up to 1.4 MW/m^2. The results show the performance and verify the applicability in a real flight environment.

Synchronized measurement of instantaneous convective heat flux and velocity fields in wall-bounded flows

This work presents an experimental setup for acquiring synchronized wall and flow field measurements in a turbulent boundary layer (TBL). Instantaneous measurements of the convective heat transfer distribution at the wall and of the flow field are carried out simultaneously, using synchronized wall-parallel planar particle image velocimetry and infrared thermography. A low-thermal-inertia heated-thin-foil sensor is embedded in the wall beneath the TBL to measure the wall temperature maps with enough temporal resolution. The unsteady energy balance of the heated foil can be solved to restore the instantaneous value of the convective heat transfer coefficient on the wall. A detailed description of the sensor design is included. Furthermore, owing to the relatively low signal-to-noise ratio of instantaneous temperature fluctuation measurements, a recipe for data processing is proposed. A study to characterize the uncertainty of the experimental setup design is also performed. The overall agreement in the correlation between wall heat transfer and velocity fields with the literature supports the validity of the proposed approach. This solution is potentially interesting for flow control purposes, where sensing is performed at the wall.

Computation of Heat Flux in Hypersonic Flow with a Cartesian Mesh Using Near-Wall Resolution

A hybrid solution methodology is developed to solve laminar hypersonic flow with a Cartesian mesh approach. The hybrid grid consisting of a Cartesian mesh together with an unstructured prism layer near the wall to capture the large gradients close to the surface is used for the near wall viscous resolution. The prism layer is generated by extruding the panels formed by the intersection of the Cartesian grid and the body and is stitched with the outer Cartesian mesh away from the wall for solving axi-symmetric flows. For three dimensional flows, since the prism layer cells are not stitched to the outer Cartesian mesh, the inviscid solution is carried out first for background Cartesian mesh and later this solution is mapped to the extruded prism layer cells and laminar Navier-Stokes solution is carried out for the prism layer cells alone. The developed solver gives an unstructured prism layer solution near the wall and standard Cartesian mesh solution away from the wall. This hybrid solution methodology is validated against the experimentally measured heat flux data and could predict the heat flux with good accuracy. The present methodology thus enables the computation of a viscous solution using a Cartesian mesh based approach.

ARCTIC ARCHITECTURES: UNLEASHING ENERGY EFFICIENCY AND RESILIENCE IN EXTREME COLD REGIONS

Severe cold regions like Alaska are grappling with the challenge of adapting to unpredictable temperature fluctuations and ensuring indoor thermal comfort for occupants as well as saving energy consumption of the building. Therefore, proactive measures are crucial to adapt buildings to withstand the damaging consequences of extreme weather conditions. This study focuses on the evaluation of dynamic thermal performance in wall assemblies using composite infill panels integrated into a framing system, featuring vacuum insulated panels (VIPs). A precast residential unit located in Alaska serves as the testing ground. A comprehensive dataset collected by the Cold Climate Housing Research Center (CCHRC) from January to April 2023 in Fairbanks, AK Leveraging, including outdoor and indoor temperature, heat flux, and relative humidity measurements is utilized. The evaluation of the wall panels' dynamic thermal performance is conducted by using two key metrics of Decrement Factor (DF) and Time Lag (TL). Additionally, the study examines the correlation between temperature fluctuations, heat flux, and relative humidity. The results reveal a direct correlation between temperature fluctuations during winter conditions and heat flux, while relative humidity demonstrates an inverse correlation. This research contributes to the development of energy-efficient and resilient buildings in Arctic regions. These insights will aid decision-makers in effectively adapting to the challenges posed by extreme weather conditions associated with climate change. Ultimately, this study paves the way for the creation of buildings that can thrive in severe cold climates while ensuring optimal energy efficiency and occupant well-being.

Thermally Significant Fluid Seepage Through Thick Sediment on the Juan de Fuca Plate Entering the Cascadia Subduction Zone

AbstractWe use heat flux measurements colocated with seismic reflection profiles over a buried basement high on the Juan de Fuca plate ∼25 km seaward of the deformation front offshore Oregon to test for the presence of hydrothermal circulation in the oceanic crust. We also revisit heat flux data crossing a buried basement high ∼25 km seaward of the deformation front ∼150 km north, offshore Washington. Seafloor heat flux is inversely correlated with sediment thickness, consistent with vigorous hydrothermal circulation in the basement aquifer homogenizing temperatures at the top of the basement. Heat flux immediately above the summit of the basement highs is greater than expected solely from conduction. Fluid seepage at rates of ∼2.6–5.4 cm yr−1 in a 1–1.5 km‐wide conduit through ∼800–1,300 m thick sediment sections above these basement highs can explain these observations. Observations of thermally significant fluid seepage through sediment >225 m thick on oceanic crust are unprecedented. High sediment permeability, high fluid overpressure in the basement, or a combination of both is required to drive fluid seepage at the observed rates. We infer that rapid seepage occurs because the basement highs rise above the low permeability basal sediment with their tops protruding into the base of high permeability Nitinat or Astoria Fan sediment. Seepage from basement highs penetrating into the submarine fans can affect the thermal state of crust entering the subduction zone.

The influence of permeability anisotropy in the upper ocean crust on advective heat transport by a ridge-flank hydrothermal system

In this study, we highlight the importance of permeability anisotropy on the hydrogeological regime of a ridge-flank hydrothermal system. Our study site, North Pond, is a marine sediment pond on ∼8 Ma seafloor in the North Atlantic, and represents a low-temperature, end-member ridge-flank hydrothermal system. Previous simulations of North Pond elucidated long-standing hypotheses concerning hydrothermal fluid and heat transport in the upper volcanic crust but failed to fully explain observed patterns of seafloor heat flux in this area. Here we use variography, a geostatistical method, to quantify relations between seafloor heat-flux measurements, and coupled numerical simulations of fluid and heat flow to simulate the hydrogeologic regime. Directional variography shows that heat-flux observations are correlated along-strike of the regional crustal fabric. Three-dimensional simulations that include permeability anisotropy are able to replicate seafloor heat-flux patterns across North Pond. The simulations that result in the best match to thermal data incorporate permeability anisotropy in the horizontal plane. We find that the feedback between permeability anisotropy and the asymmetric geometry of North Pond combine to promote advective removal of heat and mass within the crustal aquifer. These findings suggest that permeability anisotropy in the oceanic crust may influence ridge-flank hydrothermal circulation more broadly.

Surface Heat Flux Measurements Based on the Non-Integer System Identification Method Using Surface Temperature Data

This paper presents an experimental approach to inversely determine surface heat flux from surface temperature data for an arbitrary body. The relation between heat flux and temperature is calibrated using the non-integer system identification method. The surface temperature is measured using an infrared camera. The approach is applied using a well-established plasma wind tunnel condition so that the results are comparable to a reference measurement with a water calorimeter. The maximum deviation between the reference and the novel heat flux measurement was 6.3% and 14.3% for two separately generated impulse responses, respectively. In conclusion, this new method allows in future applications an additional heat flux determination based on surface temperature data. Since surface temperature data is a standard measurement, only an additional calibration step is needed to standardize this heat flux measurement method.

Heat Flux Determination from Pressure Data for a Transpiration Cooling Environment

This paper describes a new heat flux gauge for application in transpiration cooled environments. The sensor is a transpiration cooled porous element and the surface heat flux is determined from the plenum pressure trace. The sensor is characterized using the non-integer system identification method. It was calibrated at four constant mass flow rates, i.e., 15, 30, 45, and 60 mg/s, of nitrogen. Toward higher mass flow rates, stronger system response is observed. The sensor performance was determined by the analysis of a well-known Gauss function-shaped heat flux profile. The deviation is below 18%. The sensor was applied to heat flux measurements in the plasma wind tunnel PWK4 at the University of Stuttgart. The stagnation point heat flux was determined to be 20 to 395 kW/m2 for decreasing mass flow rates. From this data, the local freestream mass-specific enthalpy of 10.5 MJ/kg and uncooled surface heat flux of 1042 kW/m2 were calculated based on the stagnation point heat flux reduction over the mass flow rate increase.

Research on proper sampling parameters determination approach of digital filters in small steady-state heat flux measurement

Noise fluctuation often occurs in small steady-state heat flux measurement and it significantly disturbs the measurement precision. When using the digital filters denoising the raw heat flux signals on-line, the sampling parameters, i.e. the sampling size and sampling period have an important influence on the filtering effect. In this paper, the aim is to explore an appropriate sampling parameters determination approach. Firstly, a typical small steady-state heat flux measurement apparatus was built with a refrigerator cabinet based on the reverse heating loss method. With the help of error analysis and via particle image velocimetry technique, it is inferred that the noise mainly originates from the error of the acquisition system and the flow field unsteady-state on the given measurement condition. Then, three cases are performed to explore the determination approach of appropriate sampling parameters for six digital filters with the assist of running coefficients of variation of filtering effect assessment metrics. The numerical results indicated that the sampling independent sizes for rolling average, median, moving average, Savitzky-Golay, Gaussian and Kalman filters are 9780, 3210, 3926, 3676, 3179 and 8089, respectively. The start-point has a slight impact on the sampling independent size. The raw heat flux signals with large sampling period span can capture more noise information of the time series signal. Considering the filtering effect and sampling efficiency, the moving average filter with the sampling size of 3926 and the sampling period of 3.308 s is suitable for the steady-state heat flux denoising work of refrigerator cabinet.

Structural Design of Dual-Type Thin-Film Thermopiles and Their Heat Flow Sensitivity Performance.

Aiming at the shortcomings of the traditional engineering experience in designing thin-film heat flow meters, such as low precision and long iteration time, the finite element analysis model of thin-film heat flow meters is established based on finite element simulation methods, and a double-type thin-film heat flow sensor based on a copper/concentrate thermopile is made. The influence of the position of the thermal resistance layer, heat flux density and thickness of the thermal resistance layer on the temperature gradient of the hot and cold ends of the heat flow sensor were comprehensively analyzed by using a simulation method. When the applied heat flux density is 50 kW/m2 and the thermal resistance layer is located above and below the thermopile, respectively, the temperature difference between the hot junction and the cold junction is basically the same, but comparing the two, the thermal resistance layer located above is more suitable for rapid measurements of heat flux at high temperatures. In addition, the temperature difference between the hot and cold contacts of the thin-film heat flux sensor increases linearly with the thickness of the thermal resistance layer. Finally, we experimentally tested the response-recovery characteristics of the sensors, with a noise of 2.1 μV and a maximum voltage output of 15 μV in a room temperature environment, respectively, with a response time of about 2 s and a recovery time of about 3 s. Therefore, the device we designed has the characteristic of double-sided use, which can greatly expand the scope of use and service life of the device and promote the development of a new type of heat flow meter, which will provide a new method for the measurement of heat flow density in the complex environment on the surface of the aero-engine.

Effect of thin-film sensors on local heat transfer in convective heat flux measurement

Thin-film heat flux sensors are usually attached to the interesting surface for the measurement of wall heat flux in the aerospace industry. However, the presence of the surface-mounted sensors may disturb the velocity and thermal boundary layer, alert the local wall heat flux, and induce erroneous measured results. This study numerically investigates the effect of the presence of thin-film heat flux sensors on the local flow and heat transfer. The effects of both Reynold number of the flow and the geometry of the sensor are investigated. The results show that the local convective heat transfer can be remarkably enhanced by the presence of the sensor. In particular, when the height of the thin-film heat flux sensor is within 2% of the flow boundary layer, the enhancement of the convection heat transfer caused by the heat flux sensor is independent with the inlet velocity of the flow. Finally, an empirical model for estimating the influence of the sensor on the heat transfer is established based on the numerical findings, which can provide a useful tool for practical design to evaluate and minimize the effect of the sensor on the local convective heat transfer.

Characterizing Northeast Africa Drought and Its Drivers

This study explores the drivers of drought over northeast (NE) Africa as represented by monthly ERA5 potential evaporation during 1970–2022. The comparisons with surface heat flux and A-pan measurements suggest that potential evaporation quantifies moisture deficits that lead to drought. A principal component (PC) analysis of potential evaporation has the following leading modes: PC-1 in the Nile Basin and PC-2 in the Rift Valley. Time scores were filtered and regressed onto fields of SST, netOLR, and 500 hPa zonal wind to find teleconnections, and drought composites were analyzed for anomalous structure. The results identify that cold-phase Indian Ocean Dipole (IOD) couples with the overlying zonal Walker circulation. Deep easterly winds subside at −0.1 m/s over the west Indian Ocean and NE Africa, causing desiccation that spreads westward from the Rift Valley via diurnal heat fluxes. Insights are gained on IOD modulation based on the Pacific ENSO, but long-range forecasts remain elusive.

Measurement of the turbulent heat fluxes in mixed convection using combined stereoscopic PIV and PIT

The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus giving insight into the dynamics of plumes and Taylor–Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat along the cooling plate and back to the heated bottom plate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat flux distribution reveals that Taylor–Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes.Graphical abstract

Eddy Covariance Measurements of Turbulent Heat Fluxes over a Grass-covered Surface in a Tropical Location

In this study, an Eddy Covariance (EC) system was deployed to measure the turbulent heat fluxes (sensible and latent heat fluxes) at a farmland located at Obafemi Awolowo University, Ile-Ife, Nigeria. The period of measurement was between 2016 and 2019. The surface of the measurement area was covered by grass (Axonopus fissifolius) and changes from leafy-green during the wet season to dry twigs during the dry season. The sensible heat flux, Hs obtained ranged between -0.8 and 181.9 Wm-2 while the latent heat flux, Hl varied between 0.9 and 218.0 Wm-2. The maximum daytime value of Hs peaked in January while the maximum daytime value of Hl peaked in October. The sensible heat flux was 59 % higher than the latent heat flux during the Harmattan months (January, February and December), while the latent heat flux was 63 % higher than the sensible heat flux during the wet months (April - November). The study concluded that 91 % of the energy available at the surface during the period of study, was used for evapotranspiration while only 9 % was used for sensible heating as shown by the distribution of the Bowen ratio.

Experimental Investigation of Heat Transfer in a Flat-wall Impinging Diesel Spray Flame under Diesel-like Conditions

The heat loss through the walls of Internal Combustion Engines (ICEs) needs to be minimized and improved since it has a detrimental impact on the overall thermal efficiency of the engine. This study focuses on investigating heat transfer in a flat-wall impinging diesel spray flame, simulating diesel conditions, to optimize engine parameters. The effects of factors such as various injection pressures, nozzle hole diameters, impingement distances, and oxygen concentrations are analysed in combined and individually. Experimental techniques, such as high-speed imaging, heat flux sensors, and thermocouples, are used to visualize spray flame, measure heat flux profiles and temperature distributions, respectively. Preliminary results and their implications on heat transfer and heat loss are discussed. Regarding the parametric studies investigating the effect on wall heat loss under conditions similar to those of a small diesel engine, it was observed that the transferred heat on the wall was significant in certain conditions. These results emphasize the effectiveness of manipulating the injection rate profile as a viable step to suppress the heat transfer through the wall.

Assessment of thermal environment and thermal comfort in a typical composite ice shell building

Composite ice shell buildings are a special type of architecture found in severe cold regions and have played an important role in ice and snow tourism in recent years owing to their ability to offer tourists a romantic and novel experience. We conducted a field study to examine the thermal environment of a typical composite ice shell building in Harbin, China. In the unheated hall and separate small room, the indoor air had an average temperature and relative humidity of −20.64 °C and 76.94%, and −17.71 °C and 79.57%, respectively. Surface heat flux measurement of the north-facing building envelope revealed that the thermal properties of composite and plain ice differed significantly, suggesting that composite ice may have a lower thermal conductivity. The natural ventilation rate of 48,041 m3/h corresponded to an air change rate of 6.86 h−1, making the thermal environment highly susceptible to outdoor meteorological conditions. In addition, we conducted a questionnaire survey to study the thermal sensation and thermal comfort of the tourists. The results showed that the mean thermal sensation for the foot and head were approximately “cold " and “slightly cool”, respectively. The overall and mean thermal sensation for the hand were both between “cool” and “cold”, and the mean thermal comfort was approximately “slightly uncomfortable”. An adaptive thermal sensation prediction model suitable for composite ice shell buildings was developed through the study. These findings can assist in the design and construction of thermal environments that are better suited for short-term stays and specialty tourism.

Effect of moisture release on radiation behavior of burning coal bed between enclosed confined surfaces

In this study, a numerical analysis was performed to predict the radiation characteristics of burning coal layers heated on confined surfaces in an enclosed chamber. The one-dimensional (1D) unsteady energy balance equation and radiative transfer equation (RTE) were solved simultaneously using the semi-implicit Runge–Kutta method (SIRK) and discrete ordinate method (DOM). In addition to the 1D analysis, a commercial software was used to analyze the two-dimensional (2D) radiation behavior of the burning coal layers using the DOM.The heat generation rate (relevant to the combustion rate) and particle absorption coefficient were independently determined using combustion and thermal emission experiments. The gas absorption coefficient was calculated from gray gas models to account for the change of H2O vapor along coal and gas mixture with high moisture content. The dimensionless radiative heat flux and temperature distribution were determined for the three calculation cases. Two different predictions based on 1D and 2D geometries were validated against measured heat flux and temperature data. The effect of the moisture release inside the particle layer was particularly examined. The result indicates that the condensation of H2O is probably more responsible than the absorption of H2O vapor for a reduced heat flux observed in the combustion condition of high moisture coal burning at high pressure.

Non-invasive accurate time resolved inverse battery calorimetry

Precise knowledge of the battery heat generation rate and the internal temperature rise allows accurate thermal regulation required for enabling fast charging while minimizing side reactions and avoiding thermal runaway. With the necessity of high energy density, the size of batteries is constantly increasing. Because of poor thermal conduction, the thermal lag in large batteries is significant resulting in a considerable temperature gradient within the cell. Existing calorimetry methods only account for the external heat flow measurement and the external temperature rise and therefore cannot account for the thermal lag inside the cell. Additionally, with these methods, the temporal (time-resolved) information of the heat generation rate is compromised, which makes observation and attribution of electrochemical- thermal signatures impossible. As a potential solution, various intrusive techniques such as embedded thermal sensors have been proposed in the literature. However, these techniques are limited in scope. To enable accurate measurement of time-resolved heat flux measurement in commercial batteries, we instead propose a new battery calorimetry approach by combining state-of-the-art commercial calorimetry with an inverse heat transfer algorithm. This inverse calorimetry is completely non-intrusive with no change to commercial calorimeters. The inverse calorimetry reduces the error in heat generation rate to within 10% of the actual heat generation rate compared to 50% from the lumped capacitance method (best existing method). This method enables time resolved observation of electrochemical-thermal signatures such as a negative heat generation rate due to entropy change, which could not be observed from existing calorimetry methods.

Finale: impact of the ORCHESTRA/ENCORE programmes on Southern Ocean heat and carbon understanding.

The 5-year Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA) programme and its 1-year extension ENCORE (ENCORE is the National Capability ORCHESTRA Extension) was an approximately 11-million-pound programme involving seven UK research centres that finished in March 2022. The project sought to radically improve our ability to measure, understand and predict the exchange, storage and export of heat and carbon by the Southern Ocean. It achieved this through a series of milestone observational campaigns in combination with model development and analysis. Twelve cruises in the Weddell Sea and South Atlantic were undertaken, along with mooring, glider and profiler deployments and aircraft missions, all contributing to measurements of internal ocean and air-sea heat and carbon fluxes. Numerous forward and adjoint numerical experiments were developed and supported by the analysis of coupled climate models. The programme has resulted in over 100 peer-reviewed publications to date as well as significant impacts on climate assessments and policy and science coordination groups. Here, we summarize the research highlights of the programme and assess the progress achieved by ORCHESTRA/ENCORE and the questions it raises for the future. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Experimental Analysis of Thermo-Technical Parameters of Windows Glazing in the Pavilion Laboratory

Improving the energy performance of buildings in the context of external climatic conditions and the requirements of indoor environments is a hot topic in the construction industry. It primarily concerns reducing the energy used for heating and cooling in buildings. In the EU sector, this is addressed by the Energy Performance Directive (EPBD), which is followed by relevant national standards. The energy performance of buildings is strongly influenced by the window structures that are part of the building envelope. Their influence on energy performance is represented by the heat transfer coefficient, which differs in the actual built-in window construction from the design value given by the manufacturer. In this paper, the authors deal with its measurement in situ using the heat flux measurement method. The measurement was carried out in the pavilion laboratory of the Department of Building Engineering and Urban Planning (DBEUP), Faculty of Civil Engineering (FCE), University of Zilina (UNIZA), on three window constructions of different material bases. During the measurements, surface temperatures on the glazing, heat flux density, and air temperatures were recorded in minute increments. The influence of the year-round cycle of the outdoor environment on the embedded window structures is presented and the results are presented in the conclusion of the paper.