Wall Thermal Inertia Research Articles

Mixed convection in turbulent particle-laden channel flow at Re τ = 180

A numerical investigation of mixed convection in a turbulent particle-laden channel flow is presented. Using Eulerian-Lagrangian Direct Numerical Simulations (DNSs) within the point-particle approach, the interaction between wall turbulence, particle inertia and thermal inertia, and buoyancy in the two-way coupling regime has been studied. The flow dynamics is controlled by the friction Reynolds number, Richardson number, particle Stokes and thermal Stokes numbers, and the Prandtl number. The effects of particle inertia and buoyancy on fluid and particle statistics are shown for a friction Reynolds number of 180, Stokes numbers ranging from 0.6 to 120, Richardson numbers ranging from 2.72 × 10−4 to 27.2 at a single Prandtl number, 0.71. The results indicate that the effect of particle inertia is significant on particle statistics, but not as pronounced on fluid statistics at the relatively low volume fraction considered. Particle inertia modulates the overall heat flux in a non-monotonic way.

Linear and Nonlinear Stability Analysis of Molten Salt Natural Circulation Loop

Abstract A molten salt natural circulation loop (MSNCL) has been setup to study the steady-state, transient, and stability characteristics of molten nitrate salt, mixture of sodium nitrate, and potassium nitrate in 60:40 ratios by weight. Natural circulation experiments in a temperature range of 290 °C–600 °C have been performed. A semi-analytical linear model was derived for stability analysis of rectangular natural circulation loops with conventional localized surface heating and cooling. The developed model includes the effect of wall thermal inertia, variable internal heat transfer coefficient, finite secondary side heat transfer coefficient at cooler, and heat losses for predicting the stability map. These effects are incorporated considering their significant role in modeling high-temperature molten salt-based natural circulation systems. The developed model has been first validated with the experimental data of water loop available in literature. The developed model is then validated with the experimental data generated in MSNCL. Validation of in-house developed nonlinear model has also been performed against the same experimental data. The comparison of both linear and nonlinear stability analysis with the experimental data shows good agreement and articulates the importance of various parameters which have been included in the developed model.

Mutual Influence of External Wall Thermal Transmittance, Thermal Inertia, and Room Orientation on Office Thermal Comfort and Energy Demand

Upgrades in building energy efficiency codes led to differences between buildings designed according to outdated codes and those with most recent requirements. In this context, our study investigates the influence of external wall thermal transmittance, thermal inertia, and orientation on energy demand (heating, cooling) and occupant thermal comfort. Simulation models of an office building were designed, varying (i) the thermal transmittance values (0.20 and 0.60 W/(m2K)), (ii) the room orientation (four cardinal directions), and (iii) the wall thermal inertia (approximately 60 kJ/(m2K) for low and 340 kJ/(m2K) for high thermal inertia. The energy demand for heating and cooling seasons was calculated for Ljubljana using EnergyPlus 9.0.0 software. The reduction of the external wall thermal transmittance value from 0.6 W/(m2K) to 0.2 W/(m2K) contributes to significant energy savings (63% for heating and 37% for cooling). Thermal inertia showed considerable potential for energy savings, especially in the cooling season (20% and 13%, depending on the external wall insulation level). In addition, the orientation proved to have a notable impact on heating and cooling demand, however not as pronounced as thermal inertia (up to 7% total energy demand). Comparison of the thermal comfort results showed that when internal air temperatures are identically controlled in all the rooms (i.e., internal air temperature is not an influencing factor), the external wall thermal transmittance, thermal inertia, and room orientation show negligible influence on the average occupant thermal comfort. The simultaneous achievement of thermally comfortable conditions in the working environment and low energy use can only be achieved by simultaneously considering the U-value and thermal inertia.

A Probabilistic Model for Fire Temperature Rise in High-Rise Residential Buildings under the Action of Uncertain Factors

Due to the randomness of interior combustibles, wall thermal inertia, and opening factor, the fire temperature rise in high-rise residential buildings is uncertain. This study investigated 38 urban high-rise residential buildings, created the probability density functions of fire load density, opening factor, and wall thermal inertia, and constructed random fire scenarios for urban high-rise residential buildings. On this basis, relying on the Latin Hypercube Sampling method, this study further explored the probabilistic model for fire temperature rise in urban high-rise residential buildings under the action of uncertain factors, generated the possible temperature rise curves of fires in urban high-rise residential buildings and their probability distribution, and established the most representative temperature rise curve.

Steady state performance of molten salt natural circulation loop with different orientations of heater and cooler

Molten salts are proposed as a coolant and sensible heat storage medium for Solar Thermal Power Plant (STPP) as well as fuel and coolant for Molten Salt Reactor (MSR). The objective of the present study is to investigate the steady state behavior of molten salt in natural circulation flow condition along with effect of different orientations of heater and cooler on the steady state mass flow rate. For which a Molten Salt Natural Circulation Loop (MSNCL) has been setup with molten nitrate salt (mixture of Sodium Nitrate and Potassium Nitrate in 60:40 ratio by wt.) as working fluid. Steady state experiments in four different orientations of heater and cooler [Horizontal Heater Horizontal Cooler (HHHC), Horizontal Heater Vertical Cooler (HHVC), Vertical Heater Horizontal Cooler (VHHC) and Vertical Heater Vertical Cooler (VHVC)] have been performed and data generated are compared with the steady state correlations available in open literature which are derived considering either standard friction factor correlations or friction factor correlations deduced from their experiments and theoretical studies. Based on the deviations observed in the experimental values and correlation predictions, an improved formulation has been derived. The steady state formulation has been improved by incorporating Wall Thermal Inertia (WTI) effect and Heat Loss (HL) effect considering the importance of WTI & HL in dealing with high temperature molten salt loop. The improved formulation has been compared with the experimental results for different orientations of heater and cooler. Further, a new friction factor correlation has also been deduced from the experiments for vertical heater orientation. Description of MSNCL along with validation studies performed for different steady state correlation against experimental data generated in MSNCL, are reported in this paper.

Numerical study on the impact of wall thermal inertia on maximum smoke temperature and longitudinal attenuation in long channel fires

In order to better understand the law of smoke propagation in long channel fires and enhance the fire protection of channel structure with different lining materials, a series of fire simulations were carried out by Fire Dynamics Simulator (FDS) to investigate the impact of wall thermal inertia on the maximum smoke temperature and longitudinal attenuation under the channel ceiling. Five wall materials corresponding from the foam concrete to the steel with thermal inertia ranging from 0.1 to 165 kW2•s/(m4·K2) and four heat release rates (5 MW, 7.5 MW, 10 MW and 12.5 MW) were employed. The maximum smoke temperature above the fire source and the temperature distribution under the ceiling along the channel longitudinal centerline were attained and analyzed. The results show that the maximum smoke temperature is significantly affected by the channel wall thermal property and increases with the decrease of wall thermal inertia. Based on the simulative data, a modified model taking the influence of wall thermal inertia into consideration is put forward to predict the maximum gas temperature below the ceiling. Further, the effect of channel wall thermal inertia on the longitudinal smoke temperature decay can be negligible when the thermal inertia is greater than or equal to 1.21 kW2•s/(m4·K2), however, the foam concrete wall with the thermal inertia of 0.1 kW2•s/(m4·K2) results in a smaller attenuation rate due to less heat conducted into the channel wall. Meanwhile, the smoke temperature distribution along the channel longitudinal centerline is also related to the heat release rate (HRR), and the larger the HRR, the slower the temperature decay. The investigation indicates that the temperature distribution along the longitudinal direction follows an exponential attenuation in the near fire field but presents a form of the sum of two exponential functions in the one-dimensional smoke propagation stage. Then taking the effect of wall thermal inertia and heat release rate into account, two novel models characterizing the law of longitudinal temperature attenuation were developed near fire source and in the one-dimensional smoke spread stage respectively.

Stabilization of single phase rectangular natural circulation loop of larger diameter using orifice plate

Experimental investigations on the dependence of the stability threshold on different operating procedures have been carried out in a uniform diameter rectangular loop with orifice plates. The investigations revealed that there is no unique stability threshold for the different operating procedures with orifice confirming the existence of a hysteresis region just as in the loop without orifice. The hysteresis region can be further categorized into unconditionally stable, conditionally stable and unconditionally unstable regions even with the orifice plates. The stability region is found to increase with the introduction of the orifices. The predicted dimensionless steady state flow rate with orifice is found to be in good agreement with the data. One dimensional linear stability analysis accounting for the effective length of the orifices and elbows have been used to predict the stability map. The 1-D analysis can qualitatively predict the stabilizing effect of the orifice plates. However, the stable experimental data is not well characterized by the stability map predicted by the 1-D linear stability analysis which is attributed to the 3-D nature of the flow, the heat losses and the wall thermal inertia which are neglected in the 1-D analysis.

Defining the cooling and heating solar efficiency of a building component skin: application to a modular living wall

• Ambient wet bulb temperature limits cooling potential of a building component skin. • Daytime sol–air temperature limits heating potential of a building component skin. • Cooling and heating efficiency of the skin of a building component have been defined. • The efficiency of different skin systems are comparable in a unique value. The thermal evaluation of building components composed of a base wall with a solar passive skin solution, such as a vertical/roof greenery system, ventilated façade, reflective painting, etc., is usually performed as a whole. In this research, it has been proven that, independently of the base wall thermal inertia and insulation level, the temperature of the outermost surface layer of any building component during sunny hours is mainly dependent on the ambient air temperature and relative humidity, the incident global solar radiation and the building skin behaviour. The latter assumption has been proven on the south wall of a reference building simulated with TRNSYS. The south wall properties have been varied and the building has been subjected to different climates. The assumption’s validity has been checked for twelve south wall cases: a combination of 2 thermal transmittance, 2 thermal inertia and 3 climates. Each case has been simulated for a whole year. Based on this finding and the local ambient conditions for sunny hours, the hypothetical achievable maximum and minimum temperatures for the outermost surface layer have been defined. Then, based on the outermost surface temperature experimental measurements, the cooling and heating solar efficiencies valid for any skin solution have been defined. Furthermore, the developed methodology has been applied to a vertical living wall tested for a whole year under the accuracy and quality procedure of the PASLINK method. In this way, the cooling and heating solar efficiencies were experimentally determined for this skin solution for both, the hot cold seasons. The study has shown that the cooling efficiency during the hot season is 90.8%. As expected, even during sunny summer hours, the presence of water positively affects the performance of the façade, as it brings the base wall external surface temperature close to the ambient wet bulb temperature, therefore reducing the cooling load of the building. For the cold season, the cooling efficiency was similar, at 90.3%, which means a heating efficiency of 9.7%. Again, even for sunny winter hours, the values of the external surface temperature tend towards the ambient air wet bulb temperature, resulting in an increase in the heating demand. These experimental efficiency values allow the heating or cooling behaviour of different skin solutions to be comparable with a single number that is independent of the base wall composition. In addition, independently of the base wall composition, once the experimental efficiency value of a given skin solution is known, it allows (during sunny hours) the base wall outermost surface temperature to be calculated with precision. The latter makes it possible to increase the accuracy of the estimation of the heating and cooling demands of such methods as the degree-day method.

Optimal Dispatch of Regional Integrated Heating and Power System Based on Differential Thermal Inertia Model

As the physical carrier of Energy Internet, regional integrated energy system (RIES) has become an important role for improving comprehensive energy utilization efficiency. First, a subtle thermodynamic model of buildings and water-heating network was constructed based on differential thermal inertia model. Different from the traditional single-layer wall thermal inertial model, this paper constructed multi-layer wall thermal model. Then, an optimization scheme combining electricity and heat was established. The optimization results show that, compared with the traditional single-layer wall thermal inertia model, the proposed multi-layer wall thermal inertia model has better performance. The proposed comprehensive energy optimization scheme can reduce the cost of electricity while maintaining indoor comfort, and can provide a reference for the system operation status for distribution network dispatching.

Numerical investigation of conjugate heat transfer and entropy generation of MHD natural convection of nanofluid in an inclined enclosure

PurposeThe purpose of this paper is to investigate conjugate heat transfer of natural convection and entropy generation of nanofluid in the presence of external magnetic field via numerical approach in an inclined square cavity enclosure.Design/methodology/approachControl volume finite volume method with collocated arrangement of grids was used for discretization of continuity, momentum, solid and fluid energy equations. Rhie and Chow interpolation technique was applied to avoid checkerboard problem in pressure field and the well-established SIMPLE algorithm was followed to deal with the pressure and velocity coupling. The cavity is filled with water and nanoparticles of the aluminum oxide (Al2O3). This study has been conducted for the certain pertinent parameters of the volume fraction of nanoparticle (φ = 0–0.08), the angle of inclination (ϴ = 0°–330°), the Ra number (Ra = 103–108), the solid to fluid conductivity ratio (ksf = 1–400), the Ha number (Ha = 0–80) and the wall thickness ratio (δ/L = 0–0.3).FindingsThe results indicate that averaged Nu number increases by approximately 9% by increasing volume fraction from 0.0 to 0.08. Nu increases with an increasing inclination angle to 40° and decreases abruptly in 90° because of the formation of two weaker vorticity with opposite circulation pattern intensifying the density of isotherm curves in a vertical direction. Nu increases sharply with increasing Ra more than 105. Nu also augments almost 67% by increasing ksf = 1 to ksf = 50 and remains constant by increasing ksf more than 50. Nu number reduction is almost 72% with a variation of wall thickness ratio from d/L = 0 to 0.3. Entropy generation because of fluid flow, magnetic field and heat transfer reduces linearly almost 30%, 19% and 16% by increasing volume fraction, respectively. With increasing ksf, entropy generation because of fluid flow, magnetic field and heat transfer increases asymptotically, but Bejan number decreases.Originality/valueA brief review of conducted research studies in nanofluid flow and heat transfer reveals that the effect of wall thermal inertia was not investigated in MHD natural convection of nanofluids in an inclined enclosure. The aim of the present study is to analyze conjugate heat transfer in an inclined cavity filled with water and Al2O3.

Thermal performance of a heating system combining solar air collector with hollow ventilated interior wall in residential buildings on Tibetan Plateau

Solar air heating is very promising in residential buildings on Tibetan Plateau, and effective heat storage is indispensable for continuous heating. Exterior wall is often used for the heat storage, whereas the interior wall is seldom exploited. This paper investigates a heating system combining solar air collector with hollow ventilated interior wall for residential buildings on Tibetan Plateau. A simplified model based on discrete mathematics is established by EnergyPlus and is validated for modeling this system. With this model, a case study is performed to evaluate thermal performances of this system. Moreover, influencing factors of this system are analyzed as well. Results show that compared with the other two typical systems this system increases the minimum indoor air temperature by 6.5 °C and 3.2 °C with night heating need reduced by 79.9% and 60.7%, respectively. For the interior wall material, time constant and thermal inertia are dominant. The optimum window-wall ratio for the south façade is within 0.3–0.4, while the best circulation air flow rate is within 300–500 m³/h. The outdoor air temperature has few influences on the heating performance of this system. Overall, this system is applicable to the heating of residential buildings on Tibet Plateau.

A semi-analytical model for linear stability analysis of rectangular natural circulation loops

A semi-analytical model for stability analysis of natural circulation loops (NCL) with conventional localized surface heating and cooling is derived based on linear stability analysis (LSA) methodology. Considerations for local pressure losses, wall thermal inertia and finite secondary side heat transfer coefficient at cooler are given in this model. Heat losses are also incorporated to extend the scope of the model to high temperature loops like those operating with pressurized water, molten salts and liquid metals. A new parameter space is introduced for representing the stability maps. This parameter space gives a direct representation of operating variables and focuses on stability behavior at only feasible (theoretically) operating conditions for a given loop. The model is validated against the experimental data of two rectangular natural circulation loops from literature. The stability maps generated by this LSA model are further verified by simulating the dynamic behavior of NCL at various operating conditions. This is accomplished by numerically solving the non-linear governing equations of NCLs. A study is performed to find the effect of some aspects like wall thermal inertia on stability behavior of natural circulation loops. Local pressure losses and wall thermal inertia are found to have considerable effect on the stability behavior of the loops studied.

Improved thermal transient modeling with new 3-order numerical solution for a district heating network with consideration of the pipe wall's thermal inertia

An improved thermal transient model was put forward to predict the thermal transient behavior of a long pipe. The model took particular consideration of the influences of the pipe wall's thermal inertia. Then a new 3-order numerical solution was presented to solve the proposed model. The new solution would not only preserve the sharp temperature front during the heat propagation, but also achieve fine computational accuracy even for the coarse grids. In addition, the proposed model and numerical solution could be easily coupled with enormous common hydraulic models to be available to a general district heating (DH) network. The model and solution were validated in a real DH system. The simulation results had a good agreement with the measured data. Furthermore, in order to quantify the degree of the influence of pipe wall's thermal inertia, a practical indicator was developed based on ten types of often-used pipes in the DH projects. The research results showed that, for the large pipes with diameters over DN 200, the simulation errors caused by neglecting the pipe wall's thermal inertia were no more than 10%, which meant it was unnecessary to consider the thermal inertia for larger diameter pipes during the process of modeling.

Numerical Investigation of a Coupled Passive Cooling System Earth to Air Heat Exchange and Solar Chimney in Hot Arid Zone

The papers present a theoretical work on coupled passive cooling system, earth-to air heat exchange and high inertia well insulated solar chimney attached to the building south wall with direct outlet. The system was supposed to be used for summer cooling in hot arid zone in Ghardaia city in the south of Algeria. An investigation on the effects of the systems variable parameters on the cooling efficiency were carried out for hot summer days, these variable parameters were; Pipe: buried depth, cross section, length, disposition, number, and inlet outlet cross sections. Chimney: top and bottom sections, orientation, height, inlet outlet sections and positions, chimney wall insulation and thermal inertia. A numerical simulation carried out during summer days for two similar well insulated buildings, the first one with the above mentioned passive cooling system and the second without any passive features as a reference building, the buildings were assumed located in Ghardaia hot arid climate in the south of Algeria, in order to optimize the above cited variable parameters for the two coupled passive systems.

The influence of the wall thermal inertia over a single-phase natural convection loop with internally heated fluids

This paper deals with the influence of the piping material thermal and geometrical properties on the dynamic stability of single-phase natural circulation loops. To this purpose, a semi-analytical approach is developed by adopting the tools provided by the linear analysis. By considering a generic natural circulation loop configuration with a localized heat flux and a homogenously distributed Internal Heat Generation (IHG), the governing equations (mass, momentum and energy balance) are linearized around a steady-state solution of the system and treated by means of the Fourier transform to obtain dimensionless stability maps. Moreover, in order to verify the linear analysis methodology, a numerical strategy is adopted to solve the nonlinear governing equations and to investigate the natural circulation dynamics in the time domain. In principle, both the developed approaches can be applied to any natural circulation loop configuration. In the present work, the linear and the nonlinear analyses are applied to a specific natural circulation loop geometry, namely the Horizontal Heater Horizontal Cooler (HHHC) one. In this regard, an Object-Oriented (O-O) one-dimensional model of the HHHC loop is developed. For the assessment of the O-O model, the obtained results are compared with RELAP5 and Computational-Fluid-Dynamics (CFD) time-dependent simulations.

Heat transfer of pulsating laminar flow in pipes with wall thermal inertia

The effects of wall thermal inertia on heat transfer of pulsating laminar flow with constant power density within the pipe wall are investigated theoretically. The energy equation of the fully developed flow and heat transfer is solved by separation of variables and Green's function. The effects of the pulsation amplitude and frequency, the Prandtl number and the wall heat capacity on heat transfer features characterized by temperature, heat flux and Nusselt number are analyzed. The results show that the oscillation of wall heat flux increases along with the wall thermal inertia, while the oscillation of temperature and Nusselt number is suppressed by the wall thermal inertia. The influence of pulsation on the average Nusselt number is also obtained. The pulsating laminar flow can reduce the average Nusselt number. The Nusselt number reduction of pipe flow are a little more remarkable than that of flow between parallel plates, which is mainly caused by differences in hydraulic and thermal performances of the channels.

Theoretical analysis of wall thermal inertial effects on heat transfer of pulsating laminar flow in a channel

The effects of wall heat capacity on heat transfer of pulsating laminar flow with constant heat input between two parallel plates are theoretically investigated. The analytical solution of the fully developed thermal profile under constant wall heat flux is obtained. The results show that both the fluid temperature and Nusselt number fluctuate periodically as the driven pressure pulsates. The effects of the wall thermal inertia and Prandtl number, as well as pulsation amplitude and frequency on the fluctuations are investigated. Compared with pulsating flow without considering wall thermal inertia, the fluctuation amplitude of the fluid temperature and Nusselt number is smaller while the mean temperature difference is larger. The average Nusselt number, for both considering and neglecting wall thermal inertia, is reduced by pulsating flow. The reduction is up to 10% and cannot be neglected.

Evolution of temperature and humidity in an underground repository over the operation period

Abstract The aim of the study is to describe the ambient conditions in an underground repository of nuclear waste over a 100-year-long operation period. The evolution over time of the moisture and the temperature in the ventilation network was assessed by means of numerical simulations. Condensation events are described in terms of location, frequency and flow rate. The physical conceptual model takes into account the heat and vapour advection and exchanges with walls, and heat conduction through the host rock. The results of simulations were analysed to highlight how the architectural design, the gradual extension of storage zones, the ventilation rate and the weather conditions are likely to influence the ambient conditions all along the shafts, the galleries and the storage modules. They illustrate the significant effects on ambient conditions of the wall thermal inertia, the variation in atmospheric pressure over the shaft height, and the ventilation partition in some galleries between incoming and outgoing air from storage modules.

Two-phase refrigerant flow instability analysis and active control in transient electronics cooling systems

Two-loop refrigeration systems are being explored for two-phase cooling of ultra high power electronic components. For effective and efficient thermal management of electronic systems, active control methods are desired to suppress inherent flow instabilities especially in transient applications. This paper presents a framework for the transient analysis and active control of pressure-drop flow instabilities under varying imposed heat loads. The external effects on boiling flow characteristics and the boiling oscillatory flow responses to transient heat load changes are studied. Flow instability margins can be quantitatively predicted from an analytical two-phase flow model. In addition, the effects of wall thermal inertia on flow oscillations is systematically investigated. Based on the theoretical analysis of oscillatory flow boiling of refrigerants, a set of active control schemes are developed and studied to suppress flow oscillations and to increase the critical heat flux. With the available control devices – inlet valve and supply pump – different active control schemes are studied to improve the transient two-phase cooling performance.

Thermal flywheel effects on the time varying conduction heat transfer through structural walls

Wall time varying conduction heat transfer investigations are very important for the prediction of heating and cooling loads in air conditioning practice and absolutely essential to the passive solar heating design. The walls store heat, absorb and dissipate a fraction of it and transmit the rest into the conditioned space at a later time, which depends on the wall thermal inertia. The present work aims at the description of a developed numerical model, which is validated successfully against analytical results from the literature and allows the prediction of transient and quasi-steady-state thermal behaviour of two basic structural wall design groups of a growing thermal inertia. The model allows the calculation of the time varying conduction heat flux for a wide range of progressively heavier wall designs, under the effect of time varying meteorological conditions, something which allows their design evaluation for a specific application. A drastic reduction of the daily fluctuation of the quasi-steady-state heat flux would be possible by using a broad range of heavier walls, something which can be quantified by the introduction of a dimensionless quantity defined as wall damping-out efficiency. This is attributed to the thermal inertia of a specific wall design, which is also responsible for phasing-out of the maximum heat flux. Both phenomena, which are comparatively investigated for the two groups of specified walls, are desirable particularly for the development of peak load control strategies.