Unsteady Heat Transfer Characteristics Research Articles

Innovate, insulate, integrate: Embracing circular economy in innovative insulations for reduced air conditioning expenses and carbon mitigation in fly ash brick buildings

Global demand for energy in the construction sector has increased drastically. A substantial portion of this energy belongs to fulfilling the heating, ventilation, and air-conditioning requirements of buildings, which are essential for maintaining the well-being of people. The development of zero-energy or environmentally friendly structures is required to attain a sustainable future. Strategic design of the building’s envelopes in green energy buildings can reduce air conditioning expenses. This article focuses on several insulation-integrated fly ash brick wall envelopes that have been designed to reduce CO2 emissions, reduce air conditioning expenses, and figure out payback times in warm-temperate and hot-arid regions. The degree-hour technique for heating and cooling, as well as unsteady heat transfer characteristics, are taken into consideration in this research. This is achieved by incorporating thirteen innovative insulation materials into fly ash bricks in three distinct configurations, (i.e., insulation layer near to the outer end of the fly ash block, on the middle of the fly ash block, and near to the inside end of the fly ash block). Experimental measurements were carried out to determine the thermo-physical characteristics of insulation materials. The analytical findings are confirmed through experimental validation. In hot-arid and composite climatic zones, placing an insulation layer (cloth-40/foam-60) near the outer end of the fly ash block resulted in the highest savings on air-conditioning costs of 0.89 $/m2 and 0.90 $/m2, respectively. They also achieved the highest carbon mitigation values at 17.2 and 16.8 kg/kWh. Insulation-integrated wall envelopes represent the pinnacle of energy efficiency in green building design, seamlessly combining advanced insulation materials with cutting-edge construction techniques to minimise energy consumption and maximise sustainability.

Thermal Analysis of Building Roofs with Latent Heat Storage for Reduction in Energy Consumption and CO2 Emissions: An Experimental and Numerical Research

In green energy buildings, air conditioning charges can be lowered through careful planning of the building’s envelope. This article investigates several strategically designed phase change material (PCM) roof envelopes for savings on air conditioning prices, CO2 emission abatement, and payback timeframes in hot–arid and warm-temperate climates, taking into account unsteady heat transfer characteristics, cooling, and heating degree–hours. This is accomplished by using six different PCMs–RCC (reinforced cement concrete) roof envelope cases (RCC roof with PCM layer on the outer side, RCC roof with PCM layer on the center (middle), RCC roof with PCM layer on the inside, RCC roof with PCM layers placed on the outside and center, RCC roof with PCM layers placed on the center and inside, and RCC roof with PCM layers placed on the outer side and inside) with three PCMs (FS29 (form stable mixture), HS29 (hydrated salt), and OM29 (organic mixture)). PCM thermophysical characteristics are experimentally measured. The analytical results are experimentally validated. In hot–arid and warm-temperate regions, the layer of PCM installed on the outside of the RCC with HS29 saved the most on air conditioning expenses, at 6.29 and 6.61 $/m2, respectively. They also reported the greatest carbon mitigation of 300.55 kg of CO2/year and 281.58 kg of CO2/year with the faster payback periods. PCM roof envelopes are the most energy-efficient option for green buildings.

Unsteady heat transfer characteristics of R-407C in the subcooled flow boiling stemming from the oscillating refrigerant flow rate

The objective of this experiment is to investigate the unsteady heat transfer of R-407C subcooled flow boiling (SCFB) due to the oscillating refrigerant flow rate within a narrow tube oriented horizontally for application in energy efficiency air conditioning system. The refrigerant flow rate is input in the cycle of triangular oscillations. Influences of oscillation amplitude (ΔG/G‾) and oscillation period (tp) of the periodic flow rate on SCFB heat transfer are studied. The present results disclose that either ΔG/G‾ or tp has minor impact on time-average SCFB heat transfer performance. However, obvious changes in the temporal variations of the wall temperature on the pipe (Tw) and convective heat transfer coefficient (hr) are found, leading to three heat transfer mechanisms of single phase convection, intermittent nucleate boiling, and persistent subcooled boiling for various input heat fluxes. The Tw of persistent subcooled boiling presents the reversed tendency compared to that of single phase convection due to the variation of the triangular R-407C oscillation. In the case of intermediate heat flux, the unusual boiling phenomenon, called intermittent nucleate boiling, can be obtained depending on the R-407C mass flux. The visualized bubble images are also provided to further describe the intermittent nucleate boiling.

Experimental Investigation on Ice Accretion Upon Ice Particle Impacting onto Heated Surface

An experimental study was conducted to examine the dynamic ice accretion process upon the impingement of microsized, airborne ice particles/crystals onto a heated test surface pertinent to aeroengine icing phenomena. The experimental study was conducted in a specially designed ice crystal icing test facility to generate and inject microsized ice particles into a frozen-cold airflow. The microsized ice particles were forced to impinge onto a heated test plate with controllable surface temperatures. Upon impingement of the ice particles onto the heated test surface, the dynamic ice accretion process was found to take place over the heated surface in three distinct stages: 1) an ice-melting stage at the beginning, followed by 2) an ice/water mixture formation stage, and then 3) a water refreezing stage, causing the formation of a solid ice layer accreted on the heated test surface eventually. After impinging onto the test plate, while small ice particles with spheric shapes were found to be more ready to bounce off from the test surface, large, nonspheric-shaped ice particles experienced a catastrophic fragmentation process and break up into smaller pieces with noticeable impingement residues remaining on the test surface. The formation of a liquid water film layer on the test surface due to the melting of the impinged ice particles was found to be very beneficial to make more impinged ice particles stay sticking on the test surface, resulting in a rapid growth of the water/ice layer accreted on the heated test surface. A comprehensive theoretical analysis was also performed to examine the unsteady heat transfer characteristics during the dynamic ice accretion process. The theoretic predictions of the collection efficiency of the impinged ice particles on the heated test surface and the temperature variations of the water layer at the initial ice-melting stage were found to agree well with the experimental measurement results.

Thermal insulation design of subsea vertical X-mas tree

Temperature drop is commonly observed in subsea vertical X-mas trees during shutdown. The presence of a huge temperature difference between internal crude oil and external seawater can cause severe equipment degradation of the oil flow channel (e.g., hydrate precipitation), which can block the oil flow channel and interrupt the production process. The most vulnerable parts of a subsea vertical X-mas tree tend to be components with high convective heat transfer rates, such as production modules and short joints. We proposed an innovative approach for the insulation design of underwater equipment under a shutdown condition. First, we obtained a heat transfer analysis of the tree under working conditions through computational fluid dynamics to ascertain the initial temperature condition for an unsteady-state analysis. Second, we investigated the unsteady heat transfer characteristics of the tree with an insulation layer in the shutdown state and derived the relationships between insulation duration and thickness by data analysis. We used data analysis to identify the relationship between insulation duration and thickness. Finally, we derived the empirical formula of insulation thickness for underwater equipment given the effect of environmental factors on the heat preservation effect. We performed the experiment with an oil pipeline, and the results showed that the internal oil of the equipment did not hydrate within 8 h under the shutdown condition with insulation layers.

Unsteady heat transfer characteristics downstream of a vortex generator attached on the flat plate turbulent boundary layer

Unsteady heat transfer characteristics downstream of a vortex generator attached on the flat plate turbulent boundary layer

Investigation on Heat Transfer Behaviour of Molten Salt Natural Circulation Loop using Numerical Simulations

Molten salts are used as a coolant/heat transfer fluid in various high temperature engineering systems owing to their high boiling point at low pressure. Natural circulation of molten salt is being preferred in some systems like solar thermal power plant or in some nuclear reactors. Such systems can be studied with the help of a natural circulation loop. In this work, heat transfer characteristics of Molten Salt Natural Circulation Loop (MSNCL) are studied using 3D CFD simulations. Molten Nitrate salt, NaNO3+KNO3 (60:40 ratio by weight), is used as a fluid in MSNCL. In the MSNCL, in heater section, flow is developing and also mixed convection flow regime exists. The local Nusselt number variation in heater is calculated from computed data and is compared with that from Boelter correlation. Steady state heat transfer characteristics are obtained over a wide range of Reynolds number using CFD simulations. Unsteady heat transfer characteristics in the oscillatory flow formed in MSNCL with horizontal heater configuration are also studied and are found to be different as compared to vertical heater placed in vertical heater horizontal cooler configuration.

P025 後ろ向きステップ下流の非定常熱伝達特性(メカボケーション学生研究発表セッション)

P025 後ろ向きステップ下流の非定常熱伝達特性(メカボケーション学生研究発表セッション)

Effect of Air Space Thickness within the External Walls on the Dynamic Thermal Behaviour of Building Envelopes for Energy Efficient Building Construction

This paper presents the comprehensive investigation of the effect divided air space thickness within the wall on unsteady heat transfer characteristics such as thermal transmittance, thermal admittance, decrement factor and time lag of five building material walls for energy efficient building enclosure design. The five building material composite walls such as laterite stone, mud brick, cellular concrete, dense concrete and cinder concrete with total thirty configurations were studied. A computer simulation program was developed to compute unsteady heat transfer characteristics using the cyclic admittance procedure. From the results, it is observed that the decrement factor decreases with the increase in the divided air space thickness within the composite wall for all building materials. Dense concrete was observed to be the energy efficient from the lowest decrement factor point of view among five studied building materials. Dense concrete decrement factor decreases by 23.65% for 0.02 m air space thickness compared to the conventional composite wall without air space. It is also noticed that the time lag increases with the increase in the divided air space thickness within the composite wall for all building materials. Cellular concrete was observed to be the energy efficient from highest time lag perspective among five studied building materials. Cellular concrete time lag increases by 6.23% for 0.02 m air space thickness compared to the conventional composite wall without air space. The results of the study help in designing energy efficient building enclosures.

P064 円柱の挿入によりかく乱を受ける乱流境界層の非定常熱伝達特性(メカボケーション学生研究発表セッション)

P064 円柱の挿入によりかく乱を受ける乱流境界層の非定常熱伝達特性(メカボケーション学生研究発表セッション)

Time-resolved heat transfer characteristics for steady turbulent flow with step changing and periodically pulsating flow temperatures

Experimental and numerical investigations have been carried out to study the time-resolved characteristics of heat transfer for a steady turbulent flow with step changing and periodically pulsating flow temperatures in a rectangular channel. Fast response thin-film surface thermocouples are used to measure the test plate surface temperature history. A frequently used transient heat transfer measurement method with an assumption of time-wise constant heat transfer coefficient in a convection boundary condition and the Cook–Felderman method without such assumption or restriction are used to obtain time-resolved data for the heat transfer coefficients. Correction methods are applied for the direct measurements of time varying flow temperature and surface heat flux to eliminate the influences of thermocouple thermal inertia effects. The experimental results show that the quasi-steady assumption is valid in the transient heat transfer measurement for a steady turbulent flow when the flow temperature is constant or changes very slowly with time. When the flow temperature pulsates sinusoidally even with a small frequency of 0.25Hz, the heat transfer coefficients calculated with the Cook–Felderman method exhibit a strong unsteady behavior. They pulsate intensely in the same frequency with the flow temperature and the pulsation amplitude changes with time and flow temperature frequency. The values calculated with the transient heat transfer measurement method assuming time-wise constant heat transfer coefficients (quasi-steady approach) under all the sinusoidally pulsating flow temperatures are almost the same as the values using a step changing flow temperature. In a short period around the initial time of the transient measurement, the heat transfer coefficients calculated with both methods have a decreasing behavior when the flow temperature rises rapidly. Numerical simulations are shown to describe some basic features of the observed unsteady heat transfer characteristics, but predict generally much lower amplitudes compared to the experiments.

Effect of viscous dissipation and heat source on unsteady boundary layer flow and heat transfer past a stretching surface embedded in a porous medium using element free Galerkin method

The present paper deals with the study of unsteady heat transfer characteristics of viscous fluid flow over a stretching sheet placed in a porous medium in the presence of viscous dissipation and internal heat absorption or generation have been examined. A uniform magnetic field is applied transversely to the direction of the flow. Similarity transformations are used to convert the governing time dependent nonlinear boundary layer equations into a system of non-linear ordinary differential equations which are solved numerically by element free Galerkin method. The influence of suction parameter (λ), unsteady parameter (S), Eckert number (Ec), local porous parameter (β) and heat source/sink parameter (Q) on the velocity and temperature profiles are shown graphically. The impact of physical parameters on skin friction coefficient and wall temperature gradient is shown in tabulated form. Some of the results have been compared with finite element method. Finally, excellent validation of the present numerical results has been achieved with the earlier steady state results of Grubka and Bobba and Chen for local Nusselt number NuxRex-1/2=-θ′(0) for forced convection flow on linearly stretching surface.

Development and application of a novel technique for direct heat flux measurements in turbomachinery flows

The high performance and efficiency of modern gas turbines are only possible with temperatures inside the engine exceeding the allowed material temperatures in some areas by several hundred degrees. Therefore effective cooling methods are one of the key factors for the success of these engines. In order to achieve reliable predictions of the heat load of rotor or stator blades numerous research activities were performed to understand the nature of heat transfer in complex unsteady flows. Even numerical methods have made significant progress in recent years detailed experimental data are still necessary for validation and further development of the engines and the design tools. Here a new method to directly measure the heat flux $${\dot q}$$ at the material surface and accurately determine the heat transfer coefficienth is presented. The new sensor is based on the anisotropic characteristics of single crystals and allows the determination of the time varying heat flux on the surface of a model turbine airfoil. This feature is of special interest to study the influence of periodically disturbed flow conditions on the heat transfer characteristics of cooled turbine blades. The working principle of an anisotropic heat flux (AHF) sensor is briefly described together with the design of the actual sensor used in this study. Prior to the application of the sensor in a cascade test rig, comprehensive test of the sensor, the electronics and the data acquisition system were performed using a pulsed laser beam as heat source. To test the sensor under realistic conditions a large number of sensor was installed in a test blade and heat transfer measurements were performed in a cascade test rig equipped with a spoke-wheel wake generator. The results showed good agreement in the time mean results compared with standard techniques. Additionally time resolved data could be extracted from the sensor signals providing detailed information on the unsteady heat transfer characteristics and boundary layer development.

Moving and Rotating Sphere in the Thermal Entrance Region of a Heated Pipe

Flow and unsteady heat transfer around a moving and rotating sphere is considered as it passes through the thermal entrance region of a heated pipe. The flow is incompressible and viscous with constant properties, and the heated pipe wall is kept at a constant temperature. Two Peclet numbers of 25 and 100 are considered along with two sphere-to-pipe-diameter ratios of 0.2 and 0.4. The flow is made three dimensional by an eccentric positioning of the sphere inside the pipe. The governing equations are solved by a validated numerical method which uses a finite volume formulation in a generalized body-fitted coordinate system. An overset (Chimera) grid scheme is used to resolve the two geometries of the pipe and sphere. The results are presented in terms of the steady pressure and viscous lift and drag forces on the sphere. In addition the unsteady heat transfer characteristics of the flow are presented in terms of the local and overall Nusselt number evolution around the sphere as it passes through the thermal entrance region of the pipe. Sphere heat-up is also given as the lumped mass sphere temperature as a function of distance from the thermal entrance. [S0022-1481(00)02402-6]

Measurement of Unsteady Heat Transfer on Combustion Chamber Walls of Diesel Engine.

Aiming at understanding the characteristics of unsteady heat transfer on the combustion chamber walls of diesel engine and obtaining the basic data to establish a prediction method, simultaneous multi-point measurement of unsteadey heat flux is conducted. To investigate the influence of combustion parameters on the heat transfer to the walls, a single cylinder test engine is operated under the various kinds of engine conditions, such as swirl velocity, fuel injection pressure, the diameter of fuel injection nozzle and fuel injection timing. As a result, the influences of those parameters on unsteady heat flux are clarified quantitatively and investigated in relation to the prediction of in-cylinder combustion phenomena.

TRANSIENT CONJUGATED FORCED CONVECTION HEAT TRANSFER WITH FULLY DEVELOPED LAMINAR FLOW IN PIPES

Abstract This work deals with Ike problem of transient conjugated forced convection heat transfer with fully developed laminar flow in an infinitely long pipe, in which the external surface over a finite length of pipe is subject to a constant wall temperature. The effects are presented of Pécles number, ratios of pipe radii, wall-to-fluid diffusivity, and conductivity on the interfacial heat flux. The predicted results show that the unsteady heat transfer characteristics depend strongly on the ratios of outside and inside radix, wall-to-fluid conductivity, and thermal diffusivity. Additionally, the decrease in diffusivity ratio A from 10 to 0.1 results in a longer time for the system to reach the steady state condition from τ = 6.4 to τ = 265.

Transient Laminar Mixed Convective Heat Transfer in a Vertical Flat Duct

Unsteady laminar aiding and opposing mixed convection heat transfer in a vertical flat duct is numerically investigated for an initially fully developed flow. Results indicate that unsteady heat transfer characteristics in the flow are principally determined by wall-to-fluid heat capacity ratios. Effects of the buoyancy and degree of asymmetric heating or cooling are rather insignificant. Correlation equations for the time variations of local Nusselt numbers with wall-to-fluid heat capacity ratios are proposed.

Numerical analysis of unsteady heat transfer characteristics of a Stirling engine regenerator.

The unsteady heat transfer characteristics of a Stirling engine regenerator have been theoretically studied. The matrix of the regenerator was assumed to be a homogeneous continuum which absorbs and emits thermal radiation. The coupled energy equations for gas and matrix were solved taking account of conductive, convective and radiative heat transfer. The thermal response of the regenerator has been clarified for a wide range of incident radiation, frequency, gas velocity, and heat capacity ratio. It has been demonstrated that the incident radiation and gas-phase conduction exert considerable influence on the gas and matrix temperatures, and thus on the temperature efficiency of the regenerator.

Unsteady heat transfer characteristics in a semitransparent medium heated by diffuse and collimated radiation.

Etudes theorique et experimentale des caracteristiques du transfert de chaleur transitoire dans une couche liquide semi-transparente exposee a un rayonnement lumineux diffus et/ou collimate

Unsteady heat transfer characteristics of a two dimensional laminar wall jet

An analysis is presented to investigate the transient heat transfer characteristics of a laminar plane wall jet. The assumption of incompressible fluid with constant viscosity coefficient has been made. The transient surface heat flux and details of the temperature field are obtained and have been presented graphically. An engineering estimate of the time required to reach steady state has been given. The range of Prandtl numbers investigated is from 0.01 to 1000.