Browsing by Author "S, Anish"

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Computational and Experimental Study of Solar Air Heater With Various Duct Cross-Sections and Artificial Roughness
(National Institute of Technology Karnataka, Surathkal, 2022) K, Nidhul; Yadav, Ajay Kumar; S, Anish
Thermo-hydraulic performance and exergetic efficiency of solar air heater (SAH) with various duct cross-sections and artificial roughness have been investigated using numerical and experimental methodology. The RNG k- model with enhanced wall treatment is employed to study the turbulent flow behavior. Validation of the CFD results for smooth and artificially roughened SAH (triangular duct and duct with semi- cylindrical sidewalls) with theoretical correlations and experimental data indicates reasonable accuracy. In triangular duct SAH, the performance of inclined ribs and V-ribs have been studied e/D) and pitch (P/e). It is observed that V-ribs in triangular duct provides a maximum thermo- hydraulic performance parameter (THPP) of 2.01 with a 23% enhancement in exergetic efficiency compared to smooth SAH. Further, the performance of triangular duct SAH with inclined ribs in an indirect type solar dryer is studied. Dryer with ribbed triangular duct SAH exhibits a 60.4% and 55% reduction in moisture ratio for food samples robusta and nendran, respectively, for the same drying time compared to a dryer with a ribbed rectangular duct SAH. In addition, the design enhances the drying characteristics with 93.3% increase in average diffusivity coefficient for banana food samples. CFD analysis of SAH design with semi-cylindrical sidewalls and continuous W-baffles provides THPP in the range of 1.70 to 2.27. Maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, relative to conventional SAH at Re = 5000. Based on the optimum results obtained from CFD, an experimental setup for SAH with semi-cylindrical sidewalls and multiple discrete inclined baffles is fabricated. The experimental results indicate that THPP is further enhanced for discrete inclined baffles with the gap at the trailing apex, with a peak value of 2.69. This design has higher collector efficiency (55 to 70%) compared to ribbed rectangular SAH design exhibiting 30 to 55%. Further, the design exhibits higher exergetic efficiency owing to lower exergy losses and higher collector efficiency. Maximum exergetic efficiency of 2.2% is obtained at lower Re, higher than that obtained for rectangular duct SAH with a similar kind of artificial roughness. In addition, at low Re, this SAH design has a higher coefficient of performance (COP) than conventional SAH designs. Hence, a SAH design having lower number of sharp corners and artificial roughness capable of generating multiple secondary flow can enhance the heat transfer rate with higher thermo-hydraulic performance.
Computational Studies on Hemodynamics in Striaght and Bifurcated Arteries
(National Institute of Technology Karnataka, Surathkal, 2018) B, Prashantha; S, Anish
Hemodynamic behaviour of blood in the complexed arteries are closely related to the development of cardiovascular disease. Atherosclerosis is the major cause for the cardiovascular disease and is a chronic inflammatory process characterized by thickening of arterial wall cuase for the plaque development. The secondary flows generated at the complexed zone promotes the deposition of atherogenic particles on the outer walls. The formation and subsequent rupture of the plaque depends on wall shear stress (WSS) and oscillatory shear index (OSI). The focus of this present study is to understand the hemodynamics in the complexed region. Numerically, the analysis become more complex when a discrete phase is added to the continuous phase in order to understand the behaviour of atherogenic particles in a pulsatile flow environment. To understand the correlation between the discrete phase (atherogenic) particle behaviour with the characteristics of continuous phase (blood) under varying pulse frequencies in the post stenosis region of complexed geometries are difficult. Hence in simplified way study has been carried out with straight idealized models with different stenosis nature to analyse hemodynamics in the post stenosis region. Continuous phase is modelled by time averaged Navier-Stokes equations and solved by means of Pressure Implicit Splitting of Operators (PISO) algorithm. DPM has been carried out with one way coupling. The transport equations are solved in the Eulerian frame of reference and the discrete phase is simulated in Lagrangian frame of reference. The study brings out the importance of helicity in the atherosclerosis progression. Result shows that the asymmetric nature of stenosis exhibits less helical flow structure and the vortical structures are not getting transported to the downstream. Consequently the average particle residence time (PRT) of the atherogenic particles are one order higher than the symmetric stenosis model. Low PRT leads to enhanced mass transport in the arterial flow and triggers further occlusion/plaque build-up at the post-stenotic region. The extent of asymmetry in a diseased artery may be considered as a useful parameter in understanding the rate of progression of atherosclerosis.To understand the effect of pulse frequency on the hemodynamics of a stenosed arterial wall nature. For which the study has used a flexible wall nature. stenosis parts are assumed as isotropic, elastic homogeneous and incompressible. Different material properties can be assigned to each volume (part) to reflect the complexity (Healthy and diseased). For a healthy arterial wall young’s modulus (E) of 0.4MPa and Poissons ratio of 0.499 are selected. Stiffness was presented in the stenosed region (diseased) hence diseased part chosen to be four times of the healthy part with a young’s modulus of 1.6MPa. At each time step fluid and solid models solved individually using updated solution provided by the other part (Two way coupling). Due to the presence of larger curvature (thick wall) on one side, flow is pushed to one side of the arterial wall leading to higher stress and wall displacement in the asymmetric stenosis. Study has implemented a new device which aims at suppressing the development of atherosclerosis plaque by inducing helical flow structure in the carotid arterial passage (bifurcated). An idealized carotid artery model, chosen for the computational study. It comprises of three bilateral arteries namely Common Carotid Artery (CCA), Internal Carotid Artery (ICA) and External Carotid Artery (ECA). The CCA empties into a smaller ECA and a large ICA. The functional requirement of the swirl generator is to minimize the relative residence time (RRT) of the fluid layer near the endothelial wall without generating any additional pressure drop. With these criteria, the grooves are provided with triangular ribs at the inner wall of the swirl generator. The parameters taken for analysis are height of the rib, the helical angle and the number of ribs. Each of these parameters are varied systematically to understand their influence on the hemodynamics and the atherogenises. A total number of 11 different cases are analysed computationally using large eddy simulation (LES) model. It is observed that the induced helical flow redistributes the kinetic energy from the centre to the periphery. A single rib swirl flow generator proximal to the stent treated passage can generate sufficient helicity to bring down the RRT 36% without creating any additional pressure drop. The swirl flow adds azimuthal instability which increase vortex formations in the passage. The induced helical flow in the domain provokesmore linked vortices (coherent vortices), which may act as self-cleaning mechanism to the arterial wall and increase the stent life from restenosis. Study has investigated influence of pulsatile flow on the mechanical wall parameter after swirl flow induced in the bifurcated arterial passage. Comparsions are made between bifurcated fluid wall with enhanced swirl flow generator (single rib case) and the base case simulation. Arterial wall can be considerd as healthy (free from disease), isotropic, elastic homogeneous and incompressible and the Swirl generator device part can be assumed as rigid. Result shows that Vonmises stress on the interaction surface was generally found in the swirl flow inducer case behaving similar fashion way of Base case stress, which is acting on the wall. Additionally at the site of bifurcation, Vonmises stress closer to the base case. This is an evidence there is no such extra additional stress acting at the flow divergence by enhanced swirl flow in the bifurcated passage. uniformly distributed turbulent kinetic energy around the arterial periphery by induced swirl flow in the complexed geometry does not affect on the vascular injury. Study is focused on understand the effect of hemodynamics on the spatial and temporal variation of WSS and OSI using realistic models with varying degree of carotid artery stenosis. A series of scanned Computed Tomography (CT) images of four patients covering the carotid bifurcation artery with significant carotid artery stenosis, lumen and wall surface of the carotid bifurcation were obtained. The lumen portion was traced out from the CT image using an in-house software (ImgTracerTM, courtesy of AtheroPoint, Roseville, CA, USA). The output of the ImgTracerTM is the x-y coordinates in terms of pixels. This was further converted into millimetre (based on the resolution) and then feed into a commercially available 3D geometry modeller ICEM CFD to prepare the required polylines upon which the surface will be created. Smoothening of the surface was done using 3D modelling software CATIA. A finite volume based CFD method was utilized to understand the hemodynamics in pulsatile flow conditions. It was observed that, high stenosis models occupied a large value of normalized WSS in the ICA whereas they had smaller values of normalized WSS in the CCA. Forclinical use, present study recommend using the spatial average value of oscillatory shear rather than the maximum value for an accurate knowledge about the severity of stenosis. The bulk flow hemodynamics is represented with the direction of resultant vorticity. It reveals that, after the bifurcation zone a change of spin happens with the resultant vorticity due to the secondary flows originated from the inner wall of the bifurcation zone. Additionally, we propose the use of limiting streamlines as a novel and convenient method to identify the disturbed flow region that are prone to atherogenesis. Study has been numerically tests the optimized conceptual design of a device (swirl generator) that generate helical flow structure in the patient specific carotid artery models. The length of the helicity generator is three times the CCA diameter and it is placed five diameter distal (-5D) to the bifurcation zone in the CCA passage. At the inlet and outlets sufficient length have been provided to enhance the fully developed flow. As a result of the helical flow movement inside the arterial passage, the kinetic energy has been redistributed from the centre of the arterial passage to the periphery. This helps in washes out the recirculation regions near the bifurcation passage. Hence fluid residence time decreases with induced helical flow in the arterial passage.
Experimental Study of Natural Composite Desiccant-Based Dehumidification System
(National Institute Of Technology Karnataka, Surathkal., 2024) Dasar, Sangappa R; S, Anish; Yadav, Ajay Kumar
This study investigates the sorption and desorption characteristics of a natural com- posite desiccant based on dried cow dung (DCD). The first part of the study focuses on finding out an effective binding material for DCD. For this Polyvinyl Pyrrolidone (PVP) and clay are selected as binders. The moisture uptake capacity of composite desiccants is measured with an isotherm experiment under different DCD-to-binder ratios. Based on their isotherms, composite desiccants are chosen for the study under different humid conditions and compared with available literature data. Brunauer-Emmett-Teller and Barrett-Joyner-Halenda analyses are carried out to understand the physical properties of DCD, DCD+PVP (3:1 ratio) and DCD+Clay (3:1 ratio). Total heat load reduction, exergy efficiency and power required for these dehumidification systems are calculated for different inlet conditions. Desorption characteristics are tested at 328 K and 6% RH. Results show the maximum moisture uptake capacity of DCD and DCD+PVP as 9.87 and 9.01 g/100 g, respectively. The maximum exergy efficiency of the DCD+PVP dehumidification system is found to be 55%. The desorption time for DCD+PVP desic- cant is 17 minutes, which is 4 and 2 minutes higher compared to DCD, and DCD+Clay, respectively. In the second phase of the study, a natural composite desiccant, in which the unuti- lized portion of the spherical desiccant material is replaced with a metallic ball, is pro- posed. Stainless steel balls with a diameter of 4.75 and 6.35 mm are used to make different thickness ratios (TR = 1, 0.525, and 0.365) of metal-embedded natural com- posite desiccants (MENCDs). The natural composite desiccant is prepared from dried cow dung and polyvinyl pyrrolidone with a ratio of 3:1. Experiments are conducted to find theoptimumthicknessratioofMENCDs.Thetotalmoisturesorption,moisture sorption rate,totalheatloadreduction,andexergyefficiencyofthesedehumidification systems areinvestigatedunderdifferentrelativehumidities(RH=65%,75%and85%), and ataconstanttemperatureandvelocity.Desorptioncharacteristicsaretestedat328 K and5%RH.ThemaximummoistureupatakecapacityofMENCDswithaTRof 0.365 isfoundtobe11.84g/100g,whichis17%highercomparedtonaturalcomposite desiccants (i.e.,TR=1)at85%RH,whereas,thetotalmoisturesorptionrateis0.4 g/100 g·min, whichis20.57%higherforTRof0.365comparedtoTR=1.Themois- ture desorptionrateforTR=0.365is16.66%highercomparedtoTR=1.Thesystems exhibitanaverageexergyefficiencyof60%.However,whenemployingcompositedes- iccants withaTRof0.365,theiraverageexergyefficiencyimprovesby9.6%compared to thesystemsoperatingwithTR=1.Furthermore,theaveragereductionintotalheat load withTR=0.365is24%highercomparedtothoseutilizingTR=1. Further studiesarecarriedouttoreducethepressurelossacrossthedehumidifica- tion bedandincreasethemoisturesorptioncapacity.Toachievethisthedehumidifi- cation bedisdesignedwithstaggeredhexagonalaluminiumchannels(SHACs).The natural compositedesiccant(NCD)waspreparedbycoatingthemixtureontothechan- nels witha2:1ratioofDCD:PVPduetobettercoatability.Theresultsofthestudyshow that theNCD-coatedSHACsdehumidificationsystemhadahighmoisturesorptionca- pacity,withmaximummoisturesorptionvaluesrangingfrom8.34to14.31g/100gat differentRHandtemperatureconditions.Thesystemalsoshowsanaveragemoisture sorption rateof0.26g/100g·min andadesorptionrateof0.51g/100g·min. Further- more, themoistureflowdesignoftheNCD-coatedSHACsbedresultsinalow-pressure drop of0.13kPa, whichissignificantlylowerthantheNCD-packedbed.
An Investigtion Into The Aerodynamic Behaviour of A Compressor Cascade In A Droplet Laden Flow
(National Institute Of Technology Karnataka Surathkal, 2023) Narayanan, Deepak; S, Anish
The injection of water droplets during the compression process is a well- accepted technique for augmenting the power output from a gas turbine engine. Several researches have been carrying out in this area of ‘wet compression (overspray)’ in order to understand its impact on the aerodynamic performance of the compressor since it is a newly introduced concept in this field. Other hand boundary layer suction cavity on the blades is already a well-established technique in the field of turbo machines. The suction cavity helps to minimize boundary layer growth and helps to delay the flow separation. Hence the overall aerodynamic performance of the system would be increased. However the concept of incorporation of boundary layer suction with wet compression technology would be a new method. The first part of this study analyzes the effect of water injection on the aerodynamic performance of a linear compressor cascade. Studies have been conducted to investigate the impact of incidence angles, water droplet size, and injection ratios (IR) on the stalling characteristics of the compressor blade. Comparative analyses have been made with the dry case. Primarily, the studies have been carried out numerically using RANS simulations. The experimental analysis has also been carried out using flow visualization techniques. The study reveals notable flow modifications in the separated flow region under wet compression. For positive incidence angles, the total loss coefficient considerably decreases at the compressor’s downstream side whereas, wet compression increases the overall pressure losses inside the blade pathway at negative incidence angles. Studies with droplet size and injection ratio reveal the possibility of an optimum value for these parameters for wet compression. The flow visualization studies help to understand the water film formation and its propagation over the blades at negative and positive incidence angles. In the second part of the study, an active flow control mechanism (suction slots) is used to stabilize the boundary layer flow. Suction slots are provided to control the corner separation of the axial compressor cascade. Studies are carried out in a droplet laden flow, and comparisons are made with dry air conditions. Numerical simulations have been carried out to investigate the effect of different suction slot configurations on the iloss coefficient of the cascade. Four different slot configurations are tested near endwall slot (NES), near midspan slot (NMS), full span slot (FSS), and combined full span- endwall slot (FEWS). It was observed that the suction slot placed on the suction surface of the blade could successfully reduce the flow separation. However, the flow field at other portions of the span deteriorated as a result of this. Full-span suction (FSS) scheme on the suction surface removed boundary layer separation in the middle of the blade while greatly enhancing flow uniformity close to the end wall. Despite the improvement in flow uniformity using the full span suction scheme, a three- dimensional corner separation still existed due to the strong cross-passage pressure gradient. The combined FEWS configuration could further reduce the separation, and the total pressure loss coefficient was reduced significantly by 26 %.