Heat Input Rate Research Articles

Numerical and experimental validation of a new hybrid model for vertical ground heat exchangers

A numerical and experimental validation of a new simplified model to simulate single U-tube ground heat exchangers is presented in this paper. The model is based on the use of the electrical analogy to model heat transfer within the borehole and thermal response factors (short and long time-step g-functions) to estimate heat flow to the surrounding ground. The substitution of ground nodes with short and long-term g-functions allows the simulation with short time steps, keeping the possibility of simulating periods of several years. The model has been validated experimentally by using the reference data sets drawn from two tests (constant heat input rate and interrupted tests) made under controlled laboratory conditions. The results have also been compared with the simulated data from a detailed Computational Fluid Dynamics model of the experimental system. A very good agreement has been observed for the modeled outlet fluid and borehole temperatures, with root mean square and relative error values smaller than 0.2°C and 0.3%, respectively.

Wire-feed additive manufacturing of metal components: technologies, developments and future interests

Wire-feed additive manufacturing (AM) is a promising alternative to traditional subtractive manufacturing for fabricating large expensive metal components with complex geometry. The current research focus on wire-feed AM is trying to produce complex-shaped functional metal components with good geometry accuracy, surface finish and material property to meet the demanding requirements from aerospace, automotive and rapid tooling industry. Wire-feed AM processes generally involve high residual stresses and distortions due to the excessive heat input and high deposition rate. The influences of process conditions, such as energy input, wire-feed rate, welding speed, deposition pattern and deposition sequences, etc., on thermal history and resultant residual stresses of AM-processed components needs to be further understood. In addition, poor accuracy and surface finish of the process limit the applications of wire-feed AM technology. In this paper, after an introduction of various wire-feed AM technologies and its characteristics, an in depth review of various process aspects of wire-feed AM, including quality and accuracy of wire-feed AM processed components, will be presented. The overall objective is to identify the current challenges for wire-feed AM as well as point out the future research direction.

Joining of galvannealed steel and aluminium alloy using controlled short circuiting gas metal arc welding process

Controlling the Fe–Al intermetallic layer thickness along the joint interface has remained a critical challenge in gas metal arc welding of galvanised steel and aluminium alloys. An attempt is presented here to join galvannealed steel and aluminium alloy sheets using a novel gas metal arc welding process that allows controlled short circuiting to reduce the rate of heat input significantly. The real time current and voltage transients during the process are monitored to estimate the rate of heat input and its influence on the formation of intermetallic phases and layer thickness. The results show that the intermetallic layer thickness can be controlled and good joint strengths can be achieved when the rate of heat input remains lesser than 130 J mm− 1 for the typical lap joint configuration considered here.

Enhancement in Cooling of Electronic Components by Nanofluids

In this study, heat transfer during spray cooling was studied experimentally using water and ZnO nanofluid. Various experiments were performed using a spray nozzle impinging fluid normal to the flat end of a copper heated surface (copper cylinder 20 mm diameter). The heat flux and surface temperature have been calculated by measuring temperature gradients along the target length under steady state conditions. In this experimental study, water flow rate was varied from 15 to 25 ml/min. In the same test conditions to compare water results with nanofluids, ZnO nanofluid was sprayed at a flow rate of 20 ml/min. It can be observed that a surface temperature 74.1 °C was obtained with maximum heat flux of 102.40 W/cm2 under the test condition for heater power 140 W and a water flow rate 25 ml/min. The use of ZnO nanofluid as a coolant is observed to increase the heat flux by about 20.2 % and decrease surface temperature of the test specimen by about 15 % at 180 W heat input and flow rate of 20 ml/min. The uncertainty in heat flux is observed to vary from 8.63 to 10.93 %.

Probing Pulsed Current Gas Metal Arc Welding for Modified 9Cr-1Mo Steel

Modified 9Cr-1Mo steels are commonly welded using gas tungsten arc welding process for its superior control over the rate of heat input and vaporization loss of the key alloying elements although the rate electrode deposition remains restricted. Recent developments in pulsed current gas metal arc welding have significantly improved its ability to enhance the rate of electrode deposition with a controlled heat input rate while its application for welding of modified 9Cr-1Mo steels is scarce. The present work reports a detailed experimental study on the pulsed current gas metal arc welding of modified 9Cr-1Mo steels. The effect of the shielding gas, welding current, and speed on the weld bead profile, microstructure and mechanical properties are examined. The results show that the pulsed current gas metal arc welding with appropriate welding conditions can provide acceptable bead profile and mechanical properties in welds of modified 9Cr-1Mo steels.

A numerical study on the effects of moving regenerator to the performance of a β-type Stirling engine

An in-house CFD code has been developed to study the effects of a moving regenerator on the performance of a β-type Stirling engine. The results include temperature contours, variations of average temperature, heat transfer rates, integrated rates of heat input and output, engine power, and efficiency. It is found that the moving regenerator acts as an effective thermal barrier between the expansion and compression chambers, resulting in significant reduction in rates of heat input and output and promotion on engine’s indicated power. However, the porous medium in the moving regenerator also increases pressure loss, and hence slightly reducing the net engine power. Overall, engine’s performance, in terms of net output power and efficiency, is largely improved by the introduction of the moving regenerator. This study also examines the mechanism on how the moving regenerator achieves these effects and the influence of porosity on engine performance.

Model-Predictive-Control (MPC) of Steam Trap Subcool in Steam-Assisted Gravity Drainage (SAGD)

For thermal technologies for heavy oil and oil sand reservoir extraction, such as cyclic steam stimulation and steam-assisted gravity drainage (SAGD), suboptimal steam conformance leads to recovery factors between 25-50%. Although preliminary research using Proportional-Integral-Derivative (PID) control in SAGD operations has proved beneficial towards steam conformance, PID control is responsive only to deviation from set-points and lacks constraint-handling capabilities. This results in suboptimal actuation signals that are sometimes unattainable. This paper summarizes research on a Model-Predictive-Controller (MPC) with proactive adjustments of steam injection rate. The steam injection rate was determined based on recursive parameter updates of a suitable time varying dynamic model describing the implicit relationship between the subcool temperature difference and the input heat rate, to achieve optimal steam conformance. Furthermore, the steam injection rate was constrained such that the pressure with which the steam impinged on the formation, called well bottom hole pressure (BHP), was below the formation fracture pressure of 4500 kPa at all times. The real time control study was made possible by establishing a bidirectional communication link between the Computer Modelling Group (CMG) STARS™, and MATLAB/Simulink software. The three-dimensional heterogeneous reservoir model, developed in STARS™ acted as a virtual plant and the MPC, developed in MATLAB/Simulink, acted as an onsite controller. Results show 35.7% improvement in oil recovery and a more efficient cumulative steam-to-oil ratio (cSOR) profile in comparison to the base case of steam injection at a constant BHP of 4000 kPa.

AStudy on the Effects of Heat Input on Size of Heat-Affected Zone (HAZ) and Mechanical Properties of AISI 1518 Grade Steel in Submerged Arc-Welding Process

Submerged arc welding is a fusion-joining process which is known for its high deposition capabilities and high heat input (HI) rate. Proper management of HI in welding is important, because power sources can be used more efficiently if one knows how the same HI can be applied to get the better results. In this paper, the influence of welding HI on width of heat-affected zone (HAZ), cooling rate (CR) and mechanical property such as hardness, toughness of weld zone (WZ) in AISI 1518 grade steel with different welding condition has been systematically investigated. The results show that increasing the welding HI could due to be the HAZ, and CR are increased and decreased, respectively. However, hardness of WZ and toughness of weldments is decreased with the increase in HI. To create a good relationship between HIs with HAZ size, CR, hardness and toughness of weld is also discussed.

Experimental investigation of heat transfer enhancement through embossed fin heat sink under natural convection

Artificial surface irregularity in the form of embossed impressions is an effective technique to enhance heat transfer rates from the surface. The present study explores the heat transfer characteristics of embossed heat sink having repeated impressions on the fin surface subjected to natural convection. Heat transfer performance of the embossed fin heat sink is expressed as the Nusselt number enhancement ratio and embossed fin effectiveness for the range of impression angle from 30° to 90° while impression pitch is considered as 12mm, 16mm and 20mm under different heat input rates. A maximum enhancement in the Nusselt number for embossed fin is found to be 2.86 corresponding to the impression angle of 45° and impression pitch of 12mm with regard to smooth fin.

Three-Dimensional Heat Transfer Analysis of Laser-Arc Hybrid Welding Process

Laser-arc hybrid welding process employs a laser beam and a welding arc concurrently for the joining of thick plates at high welding speeds. Systematic quantitative studies to understand the role of welding conditions on the rate of heat input and weld pool profile are important but rarely reported for hybrid welding process. The current work presents a three-dimensional heat transfer analysis of hybrid welding process using finite element method. The computed results are validated using experimentally measured results that are reported in the independent literature. The model is used further to examine the effect of welding speed, laser and arc power, and the separation distance between laser beam and welding arc on the weld pool profile. The computed results show that the weld width will reduce with increase in welding speed and the separation distance while the penetration increases with the laser power.

Influence of Heat Input in Automatic GMAW: Penetration Prediction and Microstructural Observation

In the present work, the influence of heat input is studied on depth of penetration while depositing bead on AISI 304 stainless steel using an indigenously fabricated automatic gas metal arc welding (GMAW) movement setup. Depth of penetration and fusion zone profiles are predicted at different conditions by the bond graph modeling approach and validated using measured depth of penetration and fusion profile. The effects of heat input on toughness and metallurgical behavior are analyzed while joining AISI 304 plates using automatic GMAW. Heat input and gas flow rate significantly influence the toughness at room temperature and at −20°C. Metallurgical observations by scanning electron microscopy are carried out and it is observed that high heat input and lower cooling rate increase grain size. High heat input and rapid cooling prevent grain growth and lead to grain refinement.

A computational fluid dynamics study on the heat transfer characteristics of the working cycle of a β-type Stirling engine

A compressible CFD code has been developed to study the heat transfer characteristics of a β-type Stirling engine with a very simple design and geometry. The results include temperature contours, velocity vectors, and distributions of local heat flux along solid boundaries at several important time steps as well as variations of average temperatures, integrated rates of heat input, heat output, and engine power. It is found that impingement is the major heat transfer mechanism in the expansion and compression chamber, and the temperature distribution is highly non-uniform across the engine at any given moment. The results, especially the rates of heat transfer, are quite different from those obtained by a second-order model. The variations of heat transfer rates are much more complicated than the simple variations returned by the second-order model. This study sheds light into the complex heat transfer mechanism inside the Stirling engine and is very helpful to the understanding of the fundamental process of the engine cycle.

Steady-Periodic Acceleration Effects on the Performance of a Loop Heat Pipe

The objective of this research is to experimentally investigate the transient operating characteristics of a titanium-water loop heat pipe subjected to a combined steady-state evaporator input heat rate and a steady-periodic acceleration field. For this experimental investigation, a steady-periodic acceleration field, in the form of a sine wave, was generated using a centrifuge table. Radial acceleration peak-to-peak values and frequency of the sine wave were defined prior to conducting each experimental run and ranged from and , respectively. Evaporator input heat rate and condenser cold plate coolant temperature were varied, and , respectively. In some cases, acceleration driven forces complimented the thermodynamic forces, improving loop heat pipe dynamical performance. However, the converse was also true in that transient acceleration driven forces also appeared to counter thermodynamic forces or excite natural frequencies of the loop heat pipe. This resulted in immediate failure of the loop heat pipe to operate, delayed failure, or in some cases, the loop heat pipe operated in a stable manner but in a degraded condition.

An Investigation on the Thermal Characteristics of a Single-Effect Absorption Chiller Subjected to the Input Steam Fluctuation

This paper presents an investigation on the thermal characteristics of a single-effect absorption chiller subjected to the input steam fluctuation for an air-conditioning purpose. The absorption chiller driven by waste steam is utilized with the total capacity of 270 tons. The analyzed data are obtained by the field measurement. The results indicate that most measuring parameters appear to be nearly constant with time whereas the chilled water outlet temperature and the cooling water outlet temperature appear to have a similar wave characteristic to the mass flow rate of the input steam. The regression analysis is performed in order to obtain the sine function of these three parameters. The lagging behavior of the cooling load and the rejected heat rate behind the input heat rate from waste steam is observed due to the thermal inertia of the absorption chiller. The energy analysis shows that the average cooling load, rejected heat rate and input heat rate from waste steam is 345.78 kW, 1033.80 kW, 572.88 kW, respectively. The coefficient of performance of the absorption chiller has a high fluctuation from 0.296 to 1.524 with an average value of 0.754, resulted from the fluctuation of the mass flow rate of the input steam. The result indicates that the average value of the residual heat rate is 170.14 kW, which is approximately 10 percent of the total input heat.

A computational fluid dynamics study on the heat transfer characteristics of the working cycle of a low-temperature-differential γ-type Stirling engine

A three-dimensional compressible CFD code has been developed to study the heat transfer characteristics of a twin-power piston γ-type Stirling engine. The results include temperature contours, velocity vectors, and distributions of local heat flux along solid boundaries at several important time steps as well as variation of average temperatures, integrated rates of heat input, heat output, and engine power. It is found that Impingement is the major heat transfer mechanism in the expansion and compression chambers, and the temperature distribution is highly non-uniform across the engine volume at any given moment. This study sheds light into the complex heat transfer mechanism inside the Stirling engine and is very helpful to the understanding of the fundamental process of the engine cycle.

Numerical studies on heat release rate in a room fire burning wood and liquid fuel

Heat release rates of burning gasoline and wood fires in a room were studied by computational fluid dynamics (CFD). Version 5.5.3 of the software Fire Dynamics Simulator (FDS), which is the latest one available, was selected as the CFD simulation tool. Predicted results were compared with two sets of reported data from full-scale burning tests. In the two sets of experiments, the scenarios were set at gasoline pool fire and wood chipboard fire with gasoline respectively. The input heating rate of gasoline pool fire based on experimental measurements was used in the first set of experiments. Three scenarios G1, G2 and G3 with different grid systems were simulated by CFD. The grid system of scenario G2 gave more accurate prediction, which was then used to study the second set of experiments on wood chipboard with gasoline. The combustion model in FDS was used in wood chipboard fire induced by gasoline pool. The wood chipboard was allowed to burn by itself using the pyrolysis model in FDS. The effects of the boundary conditions on free openings for the same set of experiments were studied by three scenarios SOB1, SOB2 and SOB3. Boundary condition SOB2 gave more reliable prediction among the three boundary conditions. Two other scenarios on the effect of moisture content of wood were also studied. The predicted HRR curve was found to agree better with experiment in using SOB2.

Analytical modelling of boiling phase change phenomenon in high-temperature proton exchange membrane fuel cells during warm-up process

This paper investigates the thermal and water balance as well as the electro-kinetics during the warm-up process of a Hydrogen/Oxygen high-temperature proton exchange membrane fuel cell (HT-PEMFC) from room temperature up to the desired temperature of 180 °C. The heating strategy involves the extraction of constant current from the fuel cell, while an external heating source with a constant heat input rate is applied at the end plates of the cell simultaneously. A simple analytical unsteady model is derived addressing the boiling phase changing phenomenon in the cathode catalyst layer (CCL) and cathode gas diffusion layer (CGDL) of the cathode that occurs when the temperature of the fuel cell reaches the boiling temperature of water. Parameters such as the heat input rate, extracted current, cathode pressure and cathode stoichiometric flow ratio are varied and their effects on the temperature, liquid water fraction and most importantly, the voltage profiles with respect to time, are explored. A comparison between other existing heating strategies using the model suggests that there is insignificant improvement in warm-up time when current is extracted from room temperature considering a single cell. However, considering the solution for a typical 1-kW stack suggests that reductions in warm-up time and energy consumption can be expected. In addition, the results show that boiling phase change is found to be a key factor that affects the level of water saturation in the porous media such as the CCL and CGDL during the warm-up process, when current is extracted from the start of the process i.e. room temperature. However, the energy consumption due to boiling phase change is found to be negligible as compared to external heating input rate. The parametric studies show that the variation of heat input rate, extracted current and cathode pressure have significant effect on the cell voltage that is strongly dominated by the liquid water fraction in the porous media. On the other hand, the variation of cathode stoichiometric flow ratio is found to have minimal effect on the output cell voltage. The parametric studies also indicate that boiling phase change is present for a significant period of time under typical operating conditions.

Improvement in Welding of Austenitic Stainless Steel Thick Pipe: Metallurgical, Mechanical & Full Scale Pipe Weld Tests

The paper aims at demonstration of reduction in residual stress, distortion and sensitisation in austenitic stainless steel pipe welds. The welding processes considered are Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc welding (SMAW) along with Hot wire GTAW with narrow gap welding technique. It was shown that residual stress is significantly reduced in case of hot wire GTAW. The reduction in residual stress is due to the low heat input and high deposition rate. Lower heat input leads to higher cooling rate giving more margins on sensitization in heat affected zone (HAZ). This has been confirmed by measuring temperature and cooling rate during welding in HAZ of the weld joints. Susceptibility to sensitization of the welds has been compared by carrying out ASTM E262 Practice A and E along with Electo-Potentiokinetic Reactivation (EPR) method. The results of the tests show that values are within acceptable limit for both the cases. However, hot wire GTAW has marginally better sensitization resistance. Fatigue crack growth rate and fracture resistance behavior of the above mentioned weld joints have been compared by carrying out tests on the specimens (Compact Tension) and full scale pipe welds with crack. Fatigue crack growth rate and fracture toughness of the weld joints (hot wire GATW) is superior to conventional GTAW and SMAW. The paper presents the details of the above mentioned studies.

Effects of Palm Biodiesel Blends on Fuel Consumption in Fire Tube Boiler

Biodiesel application in boiler is basicly potential due to the combustion system tends to be simpler than compression ignition as performed in diesel engine. The various tests on combustion facilities showed that the use of biodiesel will increase the fuel consumption. This study was conducted in a fire tube boiler, with heat input rate of 60.000 kCal/hr and 3 bars of pressure, using palm biodiesel. A series of test shown that the combustion behavior in boiler was strongly possible influenced by some changes in physical properties of the fuel. The blends was varied in 10, 20, 30, 50 and 100% of biodiesel in mixture with petrodiesel. The fuel consumption was likely similar or slightly lower than petrodiesel when the fuel used in lower blends (less than 20%). In higher blends, the fuel consumption was increased up to 17%. The rate of consumption was relatively lower at average 9,4% in half load mode of the boiler operation rather than full load.

Effect of Thermosyphon cross section shape on the Heat Transfer Characteristics

Thermosyphon is considered a heat transfer device in which its operating principle depends on theevaporation and condensation of a working fluid which use the gravity forces to ensure the fluidcirculation. In the present paper, thermosyphon has been constructed to simulating the actualapplications to study the characteristics of heat transfer and its influence on the variation of crosssection shape. This paper, includes the procedure of a series experiments for three iron tubes invertical position with different cross section shape (square, rectangular and circular) having thesame hydraulic diameter and total length. Many factors have been investigate to distinguish theireffect on the heat transfer coefficient and temperature distribution on the surface. These variablesare: various cross section shape, heat input and coolant flow rate of the condenser, with themethanol as working fluid of fixed rate at operating pressure which is near to the atmosphericpressure. It has been demonstrated that from the total comparison of all shapes in the sameconditions, the performance of the square thermosyphon is higher than that of circular andrectangular thermosyphon, respectively. Also, it has been found that the highest value of the heattransfer coefficient at the square thermosyphon (1812W/m2oC) when it is compared with thepractical results of the other two types geometric shapes.