Experimental Measurements of the Volumetric Heat Transfer Coefficient between Forced Air and Sand at Reynold’s Numbers Relevant to Smouldering Combustion (original) (raw)

Small-scale forward smouldering experiments for remediation of coal tar in inert media

2020

Extended Abstract Smouldering combustion, a flameless form of combustion The main objective of this study was to model heat transfer in sand under forced air flow conditions relevant to smouldering systems. To achieve this, a new empirical correlation was developed for the volumetric heat transfer coefficient (hv) between forced air and sand at low Reynolds numbers. Nineteen experiments were conducted in a onedimensional column to evaluate the volumetric heat transfer coefficient for different air injection rates and sand particle diameters. A 1D numerical model based on local thermal non-equilibrium and Darcy flow was developed to evaluate the temperature of the solid (sand) and fluid (air) phases. The thermal properties of sand, measured with the Transient Plane Source (TPS) method (Hot Disk Thermal Constants Analyser)

Feasibility Study and Experimental Investigation of Heat and Mass Transfer in Dry and Moisturised Sand for Energy Savings

Springer Series in Geomechanics and Geoengineering, 2018

Airconditioning load and ventilation in buildings contribute more than 60% of the total energy consumption in United Arab Emirates (UAE), where the unavoidable extreme temperatures hit the region, especially during summer months. Literature survey indicates that at a depth of approximately 10 m, the soil reaches stability and it is no longer fluctuating drastically under the influence of external ambient temperatures. This relatively shallow location allows the use of soil's practical location as a possible source for sustainable energy recovery. The aim of this investigation is to explore the feasibility of using soil as a heat sink and experimentally analyse the soil heat transfer with variations in moisture saturations and heat flux rates in a testing rig. For this purpose, an axial to radial experimental analysis on Abu Dhabi soil is carried out on a lab scale test rig. This paper provides an insight into modular design of the experimental setup using spiral and U-shape heat exchangers. Experimental validation results are presented which deliver a heat profile in the sand, measured in time and depth for various heat flux rates. Moreover, thermal conductivity of the sand is estimated from experimentation and compared with literature. The results indicate presence of considerable potential for energy savings in the region.

Investigation of impingement heat transfer for air-sand mixture flow

The Canadian Journal of Chemical Engineering, 2015

Heat transfer between a heated flat plate and normal impinging gas-solid two-phase jet flow has been investigated. A single jet from a nozzle of 10 mm diameter at nozzle-to-plate distance/nozzle diameter in the range of 2 to 8 was used. Natural sand particles with average diameters of 220, 350 and 550 µm are used as a solid phase. The effect of particle size and loading ratio (mass of sand/ mass of air) at different jet velocities on impingement cooling characteristics of flat plate are investigated. The numerical simulations are performed with the ANSYS Fluent 14.7 for steady, three-dimensional, incompressible turbulent flow using Eulerian simulation for gas phase and Lagrangian simulation for sand particles. The experimental results show that the existence of sand particles decreases the Nusselt number compared to air jet flow. The single and two-phase flow experimental results are compared well with predictions when the particle reflection option is used in the simulation. The discrepancy in the local values near the stagnation point can be attributed to the complex nature of the two-phase flow at the stagnation point that includes reflection of sand particles at different angles.

Self-sustaining smouldering combustion of coal tar for the remediation of contaminated sand: Two-dimensional experiments and computational simulations

Fuel, 2015

This study presents the development and validation of a computational model which simulates the propagation of a smouldering front through a porous medium against unique experiments in coal tar and sand. The model couples a multiphase flow solver in porous media with a perimeter expansion module based on Huygens principle to predict the spread. A suite of two-dimensional experiments using coal tar-contaminated sand were conducted to explore the time-dependent vertical and lateral smouldering front propagation rates and final extent of remediation as a function of air injection rate. A thermal severity analysis revealed, for the first time, the temperature-time relationship indicative of coal tar combustion. The model, calibrated to the base case experiment, then correctly predicts the remaining experiments. This work provides further confidence in a model for predicting smouldering, which eventually is expected to be useful for designing soil remediation schemes for a novel technology based upon smouldering destruction of organic contaminants in soil.

Flame behavior in heated porous sand bed

2007

This paper reports experimental and numerical investigations on the combustion characteristics of a lean methane–air mixture in a heated porous sand bed. The porous bed consisted of sand (SiO2) particles with a mean particle diameter of 0.56 mm. The horizontally placed quartz tube was heated externally to initiate the combustion reaction in the porous bed combustor. The stabilized flame location curve as a function of averaged mixture velocity was obtained for various equivalence ratios.

Heat Transfer Characteristic of Saturated Sand in Steady Flow around a Cooling Pipe

2015

Study presented throughout this paper introduces a laboratory experimental method to observe heat transfer characteristics on the geological material that experiences a cooling process. The experiment combines flow test, cooling process, and observation of temperature change by using infra-red thermograph on saturated Toyoura Sand as the observed heat-transfer medium. Saturated sand is stored in a rectangular container that is connected with water flow and coolant pipe systems while infra-red thermograph camera observing the temperature change on the saturated sand mass. +5.0 ̊C brine solution is used as the coolant fluid in this experiment so that cooling process could not lead into frozen condition. Thermograph observation results show the obvious temperature change during cooling process. The results also indicate the effect of water flow rate to the heat transfer characteristic. Moreover, flow rate and initial temperature variations are resulted distinctive heat transfer distrib...

Thermal Conductivity of Binary Sand Mixtures Evaluated through Full Water Content Range

Soil Science Society of America Journal, 2016

Core Ideas Thermal conductivity and water content relationship based upon mixtures are distinct. Systematic change in particle fine fraction directly affects thermal conductivity. Strong correlation between density and thermal conductivity Campbell model well predicts thermal conductivity with all four parameters calibrated. Lu and Dong model well predicts thermal conductivity with both parameters calibrated. A soil's grain‐size distribution affects its physical and hydraulic properties; however, little is known about its effect on soil thermal properties. To better understand how grain‐size distribution affects soil thermal properties, specifically the effective thermal conductivity, a set of laboratory experiments was performed using binary mixtures of two uniform sands tightly packed with seven different mixing fractions over the full range of saturation. For each binary mixture, the effective thermal conductivity, λ, capillary pressure, hc, and volumetric water content, θ, w...

Characterization of Thermal, Hydraulic, and Gas Diffusion Properties in Variably Saturated Sand Grades

Vadose Zone Journal, 2016

Detailed characterization of partially saturated porous media is important for understanding and predicting vadose zone transport processes. While basic properties (e.g., particle-and pore-size distributions and soil-water retention) are, in general, essential prerequisites for characterizing most porous media transport properties, key transport parameters such as thermal conductivity and gas diffusivity are particularly important to describe temperature-induced heat transport and diffusion-controlled gas transport processes, respectively. Despite many experimental and numerical studies focusing on a specific porous media characteristic, a single study presenting a wide range of important characteristics, together with the best-performing functional relationships, can seldom be found. This study characterized five differently textured sand grades (Accusand no. 12/20, 20/30, 30/40, 40/50, and 50/70) in relation to physical properties, water retention, hydraulic conductivity, thermal conductivity, and gas diffusivity. We used measured basic properties and transport data to accurately parameterize the characteristic functions (particle-and pore-size distributions and water retention) and descriptive transport models (thermal conductivity, saturated hydraulic conductivity, and gas diffusivity). An existing thermal conductivity model was improved to describe the distinct three-region behavior in observed thermal conductivity-water saturation relations. Applying widely used parametric models for saturated hydraulic conductivity and soil-gas diffusivity, we characterized porous media tortuosity in relation to grain size. Strong relations among average particle diameter, characteristic pore diameter from soil-water retention measurements, and saturated hydraulic conductivity were found. Thus, the results of this work are useful toward better describing, linking, and predicting mass transfer and pore network properties in variably saturated porous media. Abbreviations: KC, Kozeny-Carman; SWC, soil-water characteristic; vG, van Genuchten. Vadose zone soil is essentially a dynamic multiphasic medium. The mineral fraction, which in general shows a wide textural contrast in spatial dimension, predominantly constitutes the static soil-solid phase, while the two dynamic phases, the soil-liquid phase and soil-gaseous phase, mutually share the interstitial (pore) space. Besides composing the soil functional structure, the three phases play an instrumental role in providing important ecosystem services and thereby facilitating the subsurface life-support system. For example, the solid phase, in combination with the liquid phase, is actively involved in maintaining favorable thermal regimes in the root and microbial zones by controlling heat transfer between the soil surface and biologically active deeper soil layers (de Vries, 1974). In addition, efficient exchange of gases between the plant rhizosphere and the atmosphere takes place through the soil gaseous phase, while the plant-essential water and nutrient requirements are supplied via the soil-liquid phase. Although all these key transport processes are essential to maintaining a healthy subsurface biosphere, their relative importance can be ecosystem specific; for example, the importance of subsurface thermal properties predominates in characteristically cold arctic ecosystems (Nakano and Brown, 1971) whereas Core Ideas • This study provides physical, hydraulic, and thermal properties of five sand grades. • An extended three-region thermal conductivity model is proposed. • A gas diffusivity-based variable tortuosity parameter is examined. • Useful parametric functions and descriptive models were tested.

Analysis of Heating System in FBC of Sand Reclaimer

Thermal Science and Engineering, 2018

In recent years, the foundry sector has been showing an increased interest in reclamation of used sands. Grain shape, sieve analysis, chemical and thermal characteristics must be uniform while molding the sand for better casting characteristics. The problem that tackled by every foundry industry is that of processing an adequate supply of sand which has the properties to meet many requirements imposed upon while molding and core making. Recently, fluidized bed combustors are becoming core of ‘clean wastes technology’ due to their efficient and clean burning of sand. For proven energy efficient sand reclamation processing, analysis of heating system in fluidized bed combustor (FBC) is required. The objective of current study is to design heating element and analysis of heating system by calculation of heat losses and thermal analysis of fluidized bed combustor for improving efficiency.

Characterization of Grain‐Size Distribution, Thermal Conductivity, and Gas Diffusivity in Variably Saturated Binary Sand Mixtures

Vadose Zone Journal, 2018

Core Ideas Parameterized models are relevant to applications where different porous media mixtures are used. Models include a grain‐size distribution function to describe bimodal behavior for binary mixtures. Improved model describes observed thermal conductivity–saturation relations for binary mixtures. Combined Buckingham–Penman model used to describe observed gas diffusivity–air content relations. This work is relevant for proper simulation of mixed porous media. Characterization of differently textured porous materials, as well as different volumetric porous media mixtures, in relation to mass and heat transport is vital for many engineering and research applications. Functional relations describing physical (e.g., grain‐size distribution, total porosity), thermal, and gas diffusion properties of porous media and mixtures are necessary to optimize the design of porous systems that involve heat and gas transport processes. However, only a limited number of studies provide charact...