Convective Heat Transfer Loss Research Articles

Study of engine knocking combustion using simultaneous high-speed shadowgraph and natural flame luminosity imaging

Engine knocking is a classical but significantly impactful combustion phenomenon that arouses unfailing research interests. An in-depth understanding of the knocking mechanism holds the key to the next-generation high-efficiency spark ignition engines adopting a high compression ratio. In this study, a novel 72 kHz imaging system of simultaneous shadowgraph and natural flame luminosity (NFL) was implemented on an optical engine. The key combustion parameters that affect the engine knock intensity were investigated based on the heat release analysis. The shadowgraph image velocimetry (SIV) was utilized to measure the in-cylinder flow field. The end gas autoignition process in the engine knocking combustion was studied under knock intensity as high as 18.1 bar. The results indicate that there is an “optimum” combustion phasing that produces the highest knock intensity. The cases with more end gas mass at knock onset, though having lower mean temperatures, generally produce higher knock intensity. The knock intensity can be significantly different under the same end gas mass and mean end gas temperature, possibly due to the different temperature gradients in the end gas. The simultaneous shadowgraph and NFL imaging succussed in differentiating the density variation zone in the end gas caused by the low-temperature heat release from the hot flame zone before auto-ignition. An intense density variation zone is witnessed around the center of the cylinder under higher engine knock intensity by the shadowgraph images, showing a dark region that oscillates together with the local pressure oscillations. The coupling among the local density variation, pressure oscillation, and NFL intensity oscillation is observed during the engine knocking combustion. The velocity distribution of the flow field indicates that the engine knocking combustion enhances the in-cylinder flow, which may cause more convective heat transfer loss and flow loss.

Effects of a CFD-improved dimple stepped-lip piston on thermal efficiency and emissions in a medium-duty diesel engine

Diesel piston-bowl shape is a key design parameter that affects spray-wall interactions and turbulent flow development, and in turn affects the engine’s thermal efficiency and emissions. It is hypothesized that thermal efficiency can be improved by enhancing squish-region vortices as they are hypothesized to promote fuel-air mixing, leading to faster heat-release rates. However, the strength and longevity of these vortices decrease with advanced injection timings for typical stepped-lip (SL) piston geometries. Dimple stepped-lip (DSL) pistons enhance vortex formation at early injection timings. Previous engine experiments with such a bowl show 1.4% thermal efficiency gains over an SL piston. However, soot was increased dramatically [SAE 2022-01-0400]. In a previous study, a new DSL bowl was designed using non-combusting computational fluid dynamic simulations. This improved DSL bowl is predicted to promote stronger, more rotationally energetic vortices than the baseline DSL piston: it employs shallower, narrower, and steeper-curved dimples that are placed further out into the squish region. In the current experimental study, this improved bowl is tested in a medium-duty diesel engine and compared against the SL piston over an injection timing sweep at low-load and part-load operating conditions. No substantial thermal efficiency gains are achieved at the early injection timing with the improved DSL design, but soot emissions are lowered by 45% relative to the production SL piston, likely due to improved air utilization and soot oxidation. However, these benefits are lost at late injection timings, where the DSL piston renders a lower thermal efficiency than that of the SL piston. Energy balance analyses show higher wall heat transfer with the DSL piston than with the SL piston despite a 1.3% reduction in the piston surface area. Vortex enhancement may not necessarily lead to improved efficiency as more energetic squish-region vortices can lead to higher convective heat transfer losses.

Prediction of angular distortion in austenitic stainless steel welds using finite element analysis

The present work was carried out with an aim of predicting angular distortion in AISI 304 welds fabricated using Shielded metal arc welding (SMAW) process. Since distortion results into loss of structural integrity which consequentially affects service performance of welded structures adversely, so studies on its prediction carry industrial significance to devise their preventing as well as controlling strategies. Experimental welds involved welding of 16.4 mm thick AISI 304 austenitic stainless steel plates using SMAW process using three different levels of welding heat input. Angular distortion was measured for each welded joint. Thereafter numerical analysis of the welds was done using Finite Element Method. This model was rather a simpler one as it did not consider temperature dependent properties of materials. A moving heat source was implemented to simulate the welding process which was modelled using Gaussian heat flux source. Owing to the simplicity of the model, filler material addition and multi-pass weld deposition were not considered. The whole analysis regime was divided into two steps, first a non-linear transient thermal analysis was performed to predict the temperature, and subsequently the predicted thermal load was then used as an input to perform non-linear transient structural analysis using ANSYS Workbench. To prevent the integrity of the plate in FE model, frictionless support was added as a boundary condition. For simulating the welding environment, the environment in simulation was chosen to be stagnant air and convective heat transfer loss was also considered. The numerical simulation data was then compared with the experimentally measured data. The results of this study reveal that the FE model was able to predict the angular distortion in welds to a reasonable extent, the fidelity of which can be further increased if temperature dependent property, filler metal addition and multilayer weld deposition are also taken in account.

Performance analysis of a 10 MWp utility scale grid-connected canal-top photovoltaic power plant under Indian climatic conditions

Photovoltaic modules installed on water bodies save large land areas but climatic conditions for such solar power plants, are different due to their proximity to flowing water.Thus it is important to study the performance of a canal-top solar PV plant for its reliable operation. In the present study, energetic, exergetic performance and reliability analysis of a 10 MWp grid-connected canal-top PV plant, is studied using more than 2 years data. The average monthly performance ratio, system efficiency and exergy efficiency of the plant, are found to be 0.78, 11.90% and 12.03%, respectively. PV degradation rate is found to be higher than land based power plants. The average monthly exergy efficiency, useful thermal exergy convective heat transfer loss and exergy destruction during monitoring period, are found to be 12.03%, 18.32%, 1% and 58.65%, respectively. The performance of present plant is found to be slightly lower than land based PV plants. The possible solutions to improve output power generation by canal-top PV plants are discussed. The water saved due to reduced water evaporation is found to be about 95 million litres per year.Further follow up research areas are identified to enhance the reliability of such power plants installed over water bodies.

A one-dimensional approach towards edge crack detection and mapping using eddy current thermography

Eddy current thermography (ECT) is an emerging nondestructive evaluation (NDE) technique, which is used to detect defects in metallic components. In this study, a one-dimensional (1D) heat transfer approximation is delivered to detect, locate and size surface edge cracks in bar samples using a low-power ECT experimental setup. The induction of eddy current heat generation function on one side of the sample delivers a quasi-one-dimensional heat transfer model where heat only propagates along the sample. The presented model takes into consideration convection heat transfer losses along the sample. It is demonstrated that convection heat transfer losses are considered negligible as heat propagates along a relatively high thermal conductivity metallic material such as aluminum alloys. This leads to an approximately uniform temperature gradient in a quasi-steady-state heat transfer condition along a crack-free sample. The presence of an edge crack alters the heat propagation near the crack. Accordingly, estimating the temperature gradient along a sample at a quasi-steady-state condition allows the detection and sizing of edge cracks far from the ECT excitation coil. Applying the proposed ECT inspection technique to detect edge cracks in aluminum alloys delivered an accuracy of 13 % in crack depth estimation.

Energy Conversion by Nanomaterial-Based Trapezoidal-Shaped Leg of Thermoelectric Generator Considering Convection Heat Transfer Effect

Thermoelectric generators (TEGs) can harvest energy without any negative environmental impact using low potential sources, such as waste heat, and subsequently convert that energy into electricity. Different shaped leg geometries and nanostructured thermoelectric materials have been investigated over the last decades in order to improve the thermal efficiency of the TEGs. In this paper, a numerical study on the performance analysis of a nanomaterial-based (i.e., p-type leg composed of BiSbTe nanostructured bulk alloy and n-type leg composed of Bi2Te3 with 0.1 vol % SiC nanoparticles) trapezoidal-shaped leg geometry has been investigated considering the Seebeck effect, Peltier effect, Thomson effect, Fourier heat conduction, and surface to surrounding irreversible heat transfer loss. Different surface convection heat transfer losses (h) are considered to characterize the current output, power output, and thermal efficiency at various hot surface (TH) and cold surface (TC) temperatures. Good agreement has been achieved between the numerical and analytical results. Moreover, current numerical results are compared with previous related works. The designed nanomaterial-based TEG shows better performance in terms of output current and thermal efficiency with the thermal efficiency increasing from 7.3% to 8.7% using nanomaterial instead of traditional thermoelectric materials at h = 0 W/m2K while TH is 500 K and TC is 300 K.

Effect of Convection Heat Transfer on Performance of Waste Heat Thermoelectric Generator

Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.

Parabolic Trough Collector Performance in a Hot Climate

AbstractKuwait receives annual total radiation of roughly 5,280 MJ/m2 with nearly 3,347 h of sunshine. The potential for reduction of greenhouse gas emissions motivates widespread of renewable energy systems. In the research reported in this paper, a numerical model is developed to study the effect of different collector parameters and operating conditions on the performance of parabolic trough solar collector (PTSC) in Kuwait climate. The proposed model considers all thermal losses which have been neglected in existing models. New equations were developed and used in the present model as well as reviewing the equations for convective heat transfer losses. The effects of heat conduction in the collector tube wall and mixed convection in the inner tube neglected in previous studies are included in the proposed model. In addition, a case study is presented to examine the feasibility of integrating renewable energy systems in existing buildings. Solar energy absorption system is considered in the research r...

COMPARISON OF CFD SIMULATION OF HOT AND COLD FLUID MIXING IN T-PIPE BY PLACING NOZZLE AT DIFFERENT PLACES

The project deals with the computational fluid dynamics analysis of mixed flow of hot and cold water in T- pipes and it is mainly concentrated on analytical approach to the areas where Pipes (used for flow) are mostly susceptible to damage. The simulation is done on T-pipe by placing the nozzle at three different places, to know the pressure, temperature and velocity contours throughout the flow and comparison was made. These T- pipes are mostly used in nuclear reactor cooling system to reduce the heat in the nuclear reactors by mixing hot and cold water, where the mixing will takes place efficiently. And mixed flow at the outlet will take heat from the inlet also due to convective heat transfer loss; the temperature at the boundaries of the T- pipe will be reduced. The 2D model of the pipe is made by GAMBIT and analysis is to be carried out by using K-Epsilon in FLUENT software. The models are first generated using the data and then various velocity, temperature and pressure contours are to be drawn and graphed to analyze the flow through the pipe.

Performance Analysis of Parabolic Trough Collector in Hot Climate

In most existing buildings cooling and heating loads lead to high primary energy consumption and, consequently, high CO2 emissions. These can be substantially decreased with suitable energy concepts using appropriate integrated renewable systems. In the present study a numerical model is developed to study the effect of different collector parameters and operating conditions on the performance of parabolic trough solar collector (PTSC) in Kuwait climate. The proposed model takes into consideration the thermal interaction between absorber-envelope, and envelopeenvelope for thermal radiation losses which have been neglected in existing models. A review of the equations for convective heat transfer losses was performed as well and new equations were developed and used in the present model. The effects of heat conduction in the collector tube wall and mixed convection in the inner tube, which have been neglected in previous studies, are included in the proposed model. In addition, a case study is carried out adapting parabolic trough collectors to satisfy nominal space heating load, water heating load and cooling load of a typical Kuwaiti dwelling. Finally, the environmental impact of solar heating and cooling systems under Kuwait climate conditions is investigated. Present results indicate that convection loss from the absorber tube to supporting structures is the largest among the other losses (conduction and radiation). Also, at noon time PTSC has the smallest angle of incidence and the highest efficiency and when the annulus between the receiver surface and the glass envelope is Original Research Article British Journal of Applied Science & Technology, 4(14): 2038-2058, 2014 2039 In vacuo, conduction and convection across the annulus are effectively eliminated. In addition, space heating load and domestic water heating load can be completely provided by PTSC. The minimum required collector area is about 82 m to supply cooling loads of a typical residential house under all climatic conditions in Kuwait. A CO2 emission reduction of about 12.3tonne/year can be achieved as a result of adapting parabolic trough solar collector to a typical Kuwaiti dwelling.

Enhancement of Heating Performance of Carbon Nanotube Sheet with Granular Metal

A strategy for enhancing the heating performance of freestanding carbon nanotube (CNT) sheet is presented that involves decorating the sheet with granular-type palladium (Pd) particles. When Pd is added to the sheet, the heating efficiency of CNT sheet is increased by a factor of 3.6 (99.9 °C cm(2)/W vs 27.3 °C cm(2)/W with no Pd). Suppression of convective heat transfer loss attributes to the enhanced heat generation efficiency. However, higher heating response of CNT/Pd sheet was observed compared to CNT sheet, hence suggesting that the electron-lattice energy exchange could be additional heating mechanism in the presence of granular-type particles of Pd having a diameter of 10 nm or less. CNT sheet/Pd is quite stable, retaining its initial characteristics even after 300 cycles of on-off voltage pulses and shows fast thermal responses of the heating and cooling rates being 154 and -248 °C/s, respectively.

An investigation of using various diesel-type fuels in homogeneous charge compression ignition engines and their effects on operational and controlling issues

Homogeneous charge compression ignition (HCCI) engines appear to be a future alternative to diesel and spark-ignited engines. The HCCI engine has the potential to deliver high efficiency and very low NOx and particulate matter emissions. There are, however, problems with the control of ignition and heat release range over the entire load and speed range which limits the practical application of this technology. The aim of this paper is to analyse the use of different types of diesel fuels in an HCCI engine and hence to find the most suitable with respect to operational and control issues. The single-zone combustion model with convective heat transfer loss is used to simulate the HCCI engine environment. n-Heptane, dimethyl ether and bio-diesel (methyl butanoate and methyl formate) fuels are investigated. Methyl butanoate and methyl formate represent surrogates of heavy and light bio-diesel fuel respectively. The effects of different engine parameters such as equivalence ratio and engine speed on the ignition timing are investigated. The use of internal exhaust gas recirculation is investigated as a potential strategy for controlling the ignition timing. The results indicate that the use of bio-diesel fuels will result in lower sensitivity of ignition timing to changes in operational parameters and in a better control of the ignition process when compared with the use of n-heptane and dimethyl ether.

Reduction of convective heat transfer losses from flat surfaces

The study of reduction of convective heat transfer losses from flat surfaces by means of vertical perimenter wall has been presented. The optimum screening wall height has been theoretically calculated as: (H/D) cr = 6.283/Ra 5/16. To determine the phenomenon of influence of plate screen on convective heat transfer, Nusselt-Rayleigh correlation has been given. The experimental data were in substantial agreement with the theoretical results.

The Critical Time of Exposure to Cold Environments for Prematurely Born Infants

A mathematically unsteady-state energy balance model was established for a prematurely-born infant. This model considers only the natural convection heat transfer losses and the radiative heat transfer losses. Standard mathematical relationships were used to define the skin temperature as a function of the ambient temperature and the metabolic rate as a function of the rectal temperature. (The time taken for the rectal temperature to drop to 32°C, starting from 37°C is defined as the “critical time.”) Values of the critical time have been mathematically calculated for infants of various weights and for various values of ambient temperature. It is then predicted that the critical time is strongly dependent upon the ambient temperature and is only weakly-dependent upon the weight of the infant. The values of critical time calculated in this paper should help the pediatrician in setting allowable time limits for exposure to cold for premature infants. Since the rectal temperature is one of several factors that must be controlled by the clinician, these results may assist in planning post-delivery care for the premature infant.