13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics 2017

Permanent URI for this collectionhttp://hdl.handle.net/2263/62293

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Analysis of the energetics and stability of liquid droplets on textured surfaces with square micropillars
(HEFAT, 2017) Rahman, M.A.; Goswami, A.
Numerous studies have established that roughening a hydrophobic surface can induce superhydrophobic properties on that surface. The suspended wetting state (Cassie-Baxter state) on a microtextured surface tends to collapse to a wetted state (Wenzel state) due to external stimulations. Multiple metastable Cassie-Baxter wetting state, separated by an energy barrier from Wenzel state, may also exist. In this study, 3D droplet models are developed to numerically investigate the shapes and energies of CB droplets residing on rough surfaces patterned with square pillars. A normalized form of droplet energy is used to compare the relative stabilities of metastable states. The sequence of stable droplet configurations with increasing droplet volume is analyzed for different isotropic wetting cases. Analysis reveals that wetting configuration with the most number of pillars at the drop-base suspends the biggest with higher stability compared to other configurations . In order to explore droplet energetics on distinct substrates, the pillar width and spacing are varied in simulations. For the the same drop-base area, the substrate that gives the least value of solid-fraction at the drop-base, characterized by its pillar width and spacing, suspends the biggest droplet as the most stable CB droplet compared to others.
Experimental studies on steam condensation in horizontal and vertical microtubes
(HEFAT, 2017) Sadaghiani, A.K.; Motezakker, A.; Kosar, A.
Microchannels have increasingly been used in the industry to miniaturize heat transfer equipment, improve energy efficiency, and minimize heat transfer fluid inventory. A fundamental understanding about condensation in microscale will yield far reaching benefits for the automotive and HVAC&R industries. In this study, the effect of microchannel diameter and orientation on condensation heat transfer is investigated. Steam is used as the working fluid, microtubes with inner diameters of 500 and 900 μm inner diameters, in horizontal and vertical orientations were used. The working fluid was pumped into the vapor generator from the reservoir after passing through a micro- filter. Saturated vapor was generated via electrical heating and was then led into the condensing section. Flow condensation occurred in the microtubes. The condensate leaving from the outlet of the condensing section was cooled in the cooler before flowing through the micro flow-meter. It was found that the condensation heat transfer coefficient increased with mass flux, heat flux, and vapor quality, while pressure drop increased with the mass flux and vapor quality. At low mass fluxes, it was found that the channel orientation had a considerable effect on heat transfer coefficient, while this difference diminished as mass flux increased. As mass velocity increased, differences in heat transfer coefficient are reduced.
Numerical investigation of condensation inside an inclined smooth tube
(HEFAT, 2017) Noori, Rahim Abadi S.M.A.; Meyer, Josua P.; Dirker, Jaco
In this paper the effect of inclination angle on the condensation heat transfer coefficient, pressure drop and flow regime inside a smooth tube was investigated numerically. The working fluid was R134a at a saturation temperature of 40°C. The Volume of Fluid (VOF) multiphase flow formulation was utilized to solve the governing equations. Simulations were conducted at a heat flux of 5 kW/m2, at mass fluxes of 100 – 600 kg/m2.s, and the inclination angles were varied from vertical downward to vertical upward. The simulation results were successfully validated with the experimental data. The results showed that an optimum downward inclination angle of between -30° and -15° exists, for the heat transfer coefficients. It was also found that the effect of inclination angle on the pressure drop and void fraction became negligible at high mass fluxes and vapour qualities.
Thermogravimetric determination of thermal conductivity of lumpy limestones
(HEFAT, 2017) Sandaka, G.; Specht, E.
It is known that limestone from different origins decomposed at same kiln conditions produce either hard burnt or soft burnt lime. The reason was unknown. In the previous research, thermal conductivity is found to be one of the important thermal properties which determines the calcination behavior. In this research, lumpy limestone particles of various origins have been decomposed in the laboratory furnace. Cylindrical particles have been chosen in the size range between 14 mm to 33 mm. A hole is drilled at the center of each of the particle to insert the thermocouple. The particles are decomposed in the temperature range between 900 °C and 1200°C in a tube furnace. During the decomposition weight loss as well as the core temperature of the particles are measured and recorded simultaneously. The measured core temperature and the slope of the weight loss curves have been used as input to the model that is developed in addition to the kiln conditions. This shrinking core mathematical model developed can predict the thermal conductivity of lime obtained from various limestone. The experimental and modeling methods are described in this paper. To verify the results, thermal conductivity of few of these samples are measured using laser flash apparatus. The results obtained with these two different methods were very well matched. The thermal conductivity of lime measured using thermogravimetric decomposition method with lumpy limestone decomposition is representative of the limestones those are decomposed in the industrial kilns. The thermal conductivity of lime from various limestone were found to be in the range from 0.3 to 0.85 W/(mK).
Conjugate jet impingement heat transfer investigation
(HEFAT, 2017) Nasif, G.; Barron, R.M.; Balachandar, R.
A transient numerical investigation has been carried out in this study to evaluate the effects of the conjugate heat transfer (CHT) onto the thermal characteristics due to the air and water jet impingement process. It is shown that the convective heat transfer at the fluid-solid interface is influenced by the nozzle size, boundary heat flux and thermal conductivity of the metal. The thermal characteristics from the CHT process approaches the one with no CHT process as the thermal conductivity of the metal decreases. One of the important effects of the CHT process is to redistribute the uniform boundary heat flux inside the solid and create a non-uniform heat flux boundary at the fluid-solid interface.
A study on heat transfer characteristics of fibonacci spiral microchannel heat sink
(HEFAT, 2017) Hsu, Cheng-Hsing; Jiang, Jui-Chin; Dang, Hung-Son; Nguyen, Thi-Anh-Tuyet; Chang, Ching-Chuan
A study on a Fibonacci spiral microchannel heat sink was carried out in the present work to predict thermal resistance and pressure drop of the model. The Computational Fluid Dynamics software (ANSYS Fluent 14) and Taguchi method were used to simulate and optimize the parameters of the Fibonacci spiral microchannel heat sinks. In addition, the Minitab 17 software was also used to analyze the simulated data. The result shows that the optimal design parameters can give good agreement of the overall thermal resistance and pressure drop (with the values 0.715/Wm², 15.53kPa, respectively.) were achieved for the design of the Fibonacci spiral micro-channel heat sink.
Density measurements of endothermic hydrocarbon fuels at temperatures from 235 k to 353 k and pressures up to 4.09 mpa
(HEFAT, 2017) Feng, S.; Bi, Q.C.; Pan, H.; Liu, Z.H.; Guo, Y.J.
The densities of endothermic hydrocarbon fuels were measured covering the temperature from (235.0 to 353.0) K at pressures of (0.68,1.47, 3.06 and 4.09) MPa. The densitometer is based on the attenuation theory of gamma ray with a count rate mode. When the intensity of a gamma beam passes through fuels, it decreases exponentially. According to Beer−Lambert’s law, densities of fuel were calculated through the different count rates and densities of referenced fluid. Pure hexane and a binary mixture of n-heptane and n-octane were adopted respectively to validate the reliability and accuracy of the densitometer. Results showed that the average absolute deviation(AAD) was lower than 0.32 % and the maximum absolute deviation(MAD) was within 0.67 %.
Hygroscopic inertia influence on indoor environments : moisture buffering
(HEFAT, 2017) Bairi, A.; Gomez-Arriaran, I.; Sellens, I.; Odriozola-Maritorena, M.; Perez-Iribarren, E.
In recent years great advances have been made, both in terms of regulation and technology, regarding the thermal behavior of buildings. However, a sustainable building, in addition to being energy efficient, must also ensure adequate hygroscopic performance, so as to guarantee adequate indoor air quality and comfort. As well as thermal inertia plays a very important role in the energy demand of buildings, the hygroscopic inertia of their enclosures is a regulating element in the hygroscopic balance of their interior environments, which, when properly used, can regulate the abrupt oscillations of relative humidity inside. In particular, the inner layer of the enclosures of a building interacts with the interior environment adsorbing and desorbing moisture as a function of the relative humidity of the indoor air, and therefore, that damping capacity constitutes an important term on the moisture balance of the spaces. In this paper the influence of the moisture buffering capacity of different materials on inside relative humidity is studied for different climates, ventilation rates and vapor production schemes by numerical simulation.
Exploring optimal working fluids and cycle architectures for organic rankine cycle systems using advanced computer-aided molecular design methodologies
(HEFAT, 2017) White, M.T.; Oyewunmi, O.A.; Haslam, A.J.; Markides, C.N.
The combination of computer-aided molecular design (CAMD) with an organic Rankine cycle (ORC) power-system model presents a powerful methodology that facilitates an integrated approach to simultaneous working-fluid design and power-system thermodynamic or thermoeconomic optimisation. Existing CAMD-ORC models have been focussed on simple subcritical, non-recuperated ORC systems. The current work introduces partially evaporated or trilateral cycles, recuperated cycles and working-fluid mixtures into the ORC power-system model, which to the best knowledge of the authors has not been previously attempted. A necessary feature of a CAMD-ORC model is the use of a mixed-integer non-linear programming (MINLP) optimiser to simultaneously optimise integer workingfluid variables and continuous thermodynamic cycle and economic variables. In this paper, this feature is exploited by introducing binary optimisation variables to describe the cycle layout, thus enabling the cycle architecture to be optimised alongside the working fluid and system conditions. After describing the models for the alternative cycles, the optimisation problem is completed for a defined heat source, considering hydrocarbon working fluids. Two specific case studies are considered, in which the power output from the ORC system is maximised. These differ in the treatment of the minimum heat-source outlet temperature, which is unconstrained in the first case study, but constrained in the second. This is done to replicate scenarios such as a combined heat and power (CHP) plant, or applications where condensation of the waste-heat stream must be avoided. In both cases it is found that a working-fluid mixture can perform better than a pure working fluid. Furthermore, it is found that partially-evaporated and recuperated cycles are optimal for the unconstrained and constrained case studies respectively
Simultaneous optimal control and optimal cleaning scheduling of small heat exchanger networks subject to fouling
(HEFAT, 2017) Santamaria, Federico Lozano; Macchietto, Sandro
Refinery operations are responsible for a high fraction of the energy used in the world and have a significant environmental impact on account of CO2 emissions. One of the major causes reducing their energy efficiency is fouling, the deposition of unwanted material over the surface of heat transfer units. The effects of fouling are more evident in the preheat train of the crude distillation unit where the thermohydraulic efficiency can decrease rapidly over time and many cleaning actions or other control-based mitigations alternatives have to be implemented. The optimal cleaning scheduling and optimal control problems are typically addressed separately. The former has been usually addressed using simple models and heuristics or stochastic algorithms, due to the complexity of MINLP formulations with other than unrealistically simple models. This paper presents a novel formulation and mathematical programming approach for fouling mitigation that treats simultaneously the optimal control problem of the network and the optimal cleaning scheduling, with realistic dynamic fouling models. The NLP and MINLP optimization problems are solved via deterministic optimization algorithms. Using two small examples it is shown that the simultaneous strategy has the potential to reduce operation cost by more than 10% over and above the use of individual strategies.
An experimental study on adsorption characteristics of r134a and r404a onto silicagel adsorbents
(HEFAT, 2017) Kilic, M.; Gonul, E.
The objective of this study is to evaluate adsorption characteristics of R134a and R404a on commercially available silica gel samples. A constant volume variable pressure method at different adsorption temperatures ranging from 293 to 323 K and for pressures up to 500 kPa is used in the experimental measurements. Three different type of commercially available silica gel samples were chosen as adsorbent. The isotherms obtained from the experimental measurements were presented. Adsorption parameters were evaluated from the isotherms in order to obtain correlations in the form of the Dubinin-Astakhov (DA) equation. The pressure and temperature dependence of the isosteric enthalpies of adsorption is computed with the evaluation of the measured data and the derived equations. Further, the enthalpy of adsorption data were extracted, and correlations are provided for adsorbent-adsorbate pairs obtained from the combinations of the three different silica gel specimens and two refrigerants.
Method for analysing the heat recovery potential of thermoprocessing equipment
(HEFAT, 2017) Hubner, D.; Ortwig, H.; Paulus, M.; Wild, D.
The importance of a heat recovery capability on thermoprocessing systems for increased cost-efficiency is rising steadily, along with energy costs. Accordingly, an increasing number of methods for recovering heat from exhaust gas have emerged in recent years, and the processes in question are being applied to ever more fields. For a plant-related assessment of diverse exhaust gas heat recovery solutions from an economic and/or process engineering viewpoint, it is imperative to analyze the plant-specific energy flows and to present the resulting findings in a clear-cut manner. On the basis of this analysis it is then possible to perform a plant-related evaluation and selection of the most suitable heat recovery method. The present article describes a method that facilitates an economic assessment of the heat recovery potential of thermoprocessing equipment while also permitting a comparison of the plant-specific savings potentials for a given heat recovery process. To this end, a thermodynamic model is first developed to analyze the plant-specific energy flows; this model then enables us to compute these energy flows on the basis of process data. It is further shown how the results are presented in a clearly structured fashion to serve as the basis for further investigation. Next, it is explained how suitable heat recovery solutions can be selected for various equipment results and how the associated savings potentials can be determined. The method is applied, by way of example, to the dataset of an industrial furnace and the results obtained are discussed. It is shown that the method permits an economic evaluation of diverse heat recovery solutions for different plants in day-today operation. In addition, the data analysis provides a capability to detect defective equipment components and unidentified energy flows.
Schlieren visualizations of non-ideal compressible fluid flows
(HEFAT, 2017) Conti, C.C.; Spinelli, A.; Cammi, G.; Zocca, M.; Cozzi, F.; Guardone, A.
The peculiarities of schlieren visualizations of non-ideal compressible fluid flows are analyzed with the support of experimental evidence and numerical calculation. Schlieren visualizations are performed on a vapor flow of Siloxane MDM (Octamethyltrisiloxane, C8H24O2Si3) expanding to supersonic speeds, up to a Mach number of 2, within a converging-diverging nozzle. The nozzle constitutes the test section of the Test Rig for Organic Vapors (TROVA), an experimental facility built at the Laboratory of Compressible fluid dynamics for Renewable Energy Applications (CREA Lab) of Politecnico di Milano (Italy) to investigate non-ideal expansions of complex organic vapors close to the saturation curve and the critical point, representative of blade channel flows in Organic Rankine Cycle (ORC) turbines. The schlieren system used in this work employs a double-pass configuration featuring a mercury lamp, a bi-convex lens, a cubic beam splitter, a second focusing lens and a high-speed CMOS camera. Visualizations of MDM vapor flows showed measuringrange issues: portions of the image expected to appear bright due to negative density gradients (expansion), were dark instead. This was attributed to the strong density gradients occurring in these non-ideal flows, causing refractions intense enough to deflect light onto some system aperture stop. Ray tracing calculation indeed showed that rays crossing regions of the test area with stronger gradients were intercepted by the knife frame before reaching the camera sensor. Interestingly, these measuring range issues were found to decrease as the non-ideality of the flow decreased. Moreover, when the same nozzle geometry was used for analogous testing with air, these measuring-range issues were absent or noticeably reduced. This paper analyses both experimental and numerical evidence to investigate the two aforementioned observations and provide an explanation in terms of refractive index gradient, compressibility and pressure ratio gradient trends during an expansion.
Near-wall determination of the turbulent prandtl number based on experiments, numerical simulation and analytical models
(HEFAT, 2017) Steiner, H.; Irrenfried, C.; Brenn, G.
The Reynolds-averaged computation of turbulent flow with heat transfer most commonly introduces the turbulent Prandtl number to relate the turbulent fluxes of momentum and heat. Its significant deviation from a uniform bulk flow value for high molecular Prandtl numbers requires a reliable description of this parameter for predicting accurately the heat transfer. The present study proposes a model for the near-wall variation of this important quantity for use in an analytically computed solution of heated turbulent pipe flow. The comparison of the predictions against results from Direct Numerical Simulation (DNS) and experiments proves the proposed analytical approach as a computationally efficient alternative to the much costlier numerical approach with still acceptable accuracy. The analytically obtained results do not only demonstrate the reliability of the proposed model for the near-wall behavior of the turbulent Prandtl number, but also highlight the significance of the dependence of the material properties on the temperature. Numerical simulations mostly neglect this effect to avoid a further increase of the already high computational costs associated with the discretized solution of the heated/cooled flow field.
Wall-layer model for large eddy simulation (les) of suspended sediment transport (sst) in a lab-scale turbulent open channel flow
(HEFAT, 2017) Jourabian, M.; Armenio, V.
In this study, a wall layer model under the equilibrium assumption and in the large eddy simulation (LES) framework is applied to simulate the suspension of the sand particles in a turbulent open channel flow of water. Smagorinsky model in the Euler-Euler model, which was developed in the LESCOAST code [1], is used to solve the subgrid-scale (SGS) stresses.
Role of entropy generation and field synergy on heat transfer from confined wavy wall
(HEFAT, 2017) Harikrishnan, S.; Shaligram, Tiwari
In the present study, three-dimensional numerical investigations are carried out to find the effect of confinement due to top wall on flow and heat transfer over sinusoidal wavy wall for a fixed Reynolds number (Re) of 500. For fixed amplitude and wavelength of the wavy wall, different values of mean channel height and varying shapes of top wall have been considered. The effect of introducing waviness in top wall with respect to a plane top wall has been studied for in-phase sinusoidal and out-of-phase sinusoidal configurations. Computations are carried out by using commercial software ANSYS Fluent 16.1. Flow and heat transfer characteristics are presented in terms of Nusselt number and friction factor. Influence of secondary flows developed inside the channel on heat transfer is evaluated by calculating secondary flow intensity and field synergy angle. Irreversibilities caused due to various confinements are analyzed by calculating entropy generation due to heat transfer and friction. The variation of entropy generation rate for different top wall configurations as well as channel heights have also been presented.
Performance analysis of fined pipe in air sampling smoke detection system at refrigerated warehouse
(HEFAT, 2017) Kim, Ji Tae; Oh, Jae Sung; Lee, Young Man; Ryu, Jai Young; Ryou, Hong Sun
Refrigerated warehouse is temperature-controlled environment that contain a wide range of frozen foods and dry items. However, in situation of fire, fire is rapidly spread coupled with a dry environment and high airflow by air condition system. Therefore, to minimize damage and loss in refrigerated warehouse, a fire must be detect at a very early stage. Generally, air sampling fire detection system is widely used in refrigerated warehouse. The air sampling fire detection system sampled cold air with smoke via pipe network located the underside of the ceiling. The cold air flows into laser-smoke detector chamber installed at outdoor. However, temperature of the detector chamber falls below the dew point by cold air, condensation and frosting occur and cause malfunction. Therefore, in order to prevent condensation and frosting, aluminum fin and pipes are designed to raise the temperature above the dew point. Especially, the average temperature and humidity of summer in Korea is 30℃ and 70%. Thus, the dew temperature at summer is about 21 ℃. In this study, numerical analysis is performed to investigate heat transfer characteristics according to the number of fins. The number of fins are varying 0, 2, 4 and 8 respectively. The inlet temperature of air is – 19 ℃. The pipe diameter is 10 mm and 2.5mm thickness. The geometry of fin is 22.5mm length, 1mm thickness. Also, the fin efficiency of finned pipe is compared to the bare pipe. As a result, temperature of outlet air are -3.14 ℃, -1.35 ℃, 0.1 ℃ and 2.16 ℃ and fin efficiency are increases 0.34, 0.37 and 0.41, according to the number of fins are varying 0, 2, 4 and 8 respectively. The condensation occurs when temperature is lower than dew temperature. Therefore, to satisfy the outlet temperature above the dew point, the number of fins and pipes must be increased, but the number of fins cannot be increased further due to machining and installation problems. Therefore, to increase the temperature of the outlet over the dew point, an additional heating method is required.
Computational fluid dynamics modelling and validation of head losses in pipe bifurcations
(HEFAT, 2017) Sukhapure, Kasturi; Burns, Alan; Mahmud, Tariq; Spooner, Jake
Manifold arrangements with bifurcations/trifurcations lead to head losses in a fluid system. It is necessary to predict the steady state head loss coefficients to characterise the losses arising due to these junctions. Miller [1] provides one of the most comprehensive sources for the junction loss coefficients data. However, these data were compiled from isolated research programmes without cross validation. This paper provides detailed quantitative data of sharp-edged bifurcation loss coefficients obtained using Computational Fluid Dynamics (CFD). ANSYS Design Modeller, ICEM and CFX software are used in conjunction to model and simulate flow of water through the pipe junctions. Loss coefficients at high Reynolds number (4.5 x 106) flows are investigated. These are expressed as a function of mass flow ratio, cross sectional area ratio and branch angle. In this study, 60 scenarios of pipe junction flows are investigated. The loss coefficients calculated using CFD are compared with measured values from experiments and empirical expressions reported in literature.
Subgrid-scale modeling of turbulent heat transport in forced convection at high molecular prandtl numbers
(HEFAT, 2017) Irrenfried, C.; Steiner, H.
Assuming strong similarity between the transport of momentum and heat is a common feature of most standard subgridscale models used in Large-Eddy Simulations (LES) of turbulent flow with heat transfer. In view of the limitation of this analogy to molecular Prandtl numbers near unity the present study investigates the capability of different established model concepts in predicting the subgrid-scale heat flux, when applied in a priori LES of turbulent heated flow going well beyond this parameter range, considering Prandtl numbers Pr = 1=10=20 at Reynolds number Ret = 360. The test unveils the major deficits of the constant-coefficent Smagorinsky approach due to the nonuniversiality of the used model coefficients like the turbulent subgrid-scale Prandtl number. Apart from the removal of this basic shortcoming the dynamic Smagorinsky model is shown to yield no substantially better predictions. The same holds true for the computationally more elaborate non-linear extensions introducing a tensorial diffusivity. The scale-similarity based mixed dynamic model proposed by [1] was proven to give in general the most accurate description. Some discrepancy appeared in regions with considerable net transfer of heat from the unresolved into the smallest resolved scales observed for higher Prandtl number. This suggests to include a sub-model for the presently neglected cross-scale interaction into the formulation as path for further improvement of this best evaluated approach.
Alternative design of hybrid desiccant cooling systems
(HEFAT, 2017) Nobrega, C.E.L.; Brum, N.C.L.
Traditional vapor-compression refrigeration systems handle the air cooling and dehumidification in a single process. Depending on the characteristics of the thermal load, it is not possible to simultaneously meet the cooling and dehumidification requirements, resulting in under dried or overcooled supply air. Recent studies indicate that a combination of adsorptive dehumidification and vapor-compression might result in an improved system, regarding both energy consumption and air-quality. Accordingly, the present study aims at an alternative design of a hybrid system. It is shown that such a system can provide air at the same supply condition as a vapor-compression system, requiring as much as 40% less cooling capacity. Also, the proposed system works with a 100% of air renovation, being compatible with any air quality standard.

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