An experimental investigation of a household size trigeneration

A household size trigeneration based on a small-scale diesel engine generator set is designed and realized in laboratory. Experimental tests are carried out to evaluate the performance and emissions of the original single generation (diesel engine generator); and the performances of the whole trigeneration including the diesel generator within the trigeneration system, the heat exchangers which are used to recover heat from engine exhaust, the absorption refrigerator which is driven by the exhaust heat; and the emissions from the whole trigeneration.Comparisons of the test results of two generations are also performed. The test results show that the total thermal efficiency of trigeneration reaches to 67.3% at the engine full load, comparing to that of the original single generation 22.1% only. Within the range of engine loads tested, the total thermal efficiencies of trigeneration are from 205% to 438% higher than that of the thermal efficiency of single generation.The CO₂ emission per unit (kW h) of useful energy output from trigeneration is 0.401 kg CO₂/kW h at the engine full load, compared to that of 1.22 kg CO₂/kW h from single generation at the same engine load. Within the range of engine loads tested, the reductions of CO₂ emission per unit (kW h) of trigeneration output are from 67.2% to 81.4% compared to those of single generation.The experimental results show that the idea of realizing a household size trigeneration is feasible; the design and the set-up of the trigeneration is successful. The experimental results show that the innovative small-scale trigeneration is able to generate electricity, produce heat and drive a refrigeration system, simultaneously from a single fuel (diesel) input.     

Numerical study of an exhaust heat recovery system using corrugated tube heat exchanger with twisted tape inserts

The purpose of this work is to investigate gas to liquid heat transfer performance of concentric tube heat exchanger with twisted tape inserted corrugated tube and to evaluate its impact on engine performance and economics through heat recovery from the exhaust of a heavy duty diesel generator (120 ekW rated load). This type of heat exchanger is expected to be inexpensive to install and effective in heat transfer and to have minimal effect on exhaust emissions of diesel engines. This type of heat exchanger has been investigated for liquid to liquid heat transfer at low Reynolds number by few investigators, but not for gas to liquid heat transfer. In this paper, a detail of heat transfer performance is investigated through simulations using computer software. The software is first justified by comparing the simulation results with the developed renowned correlations. Simulations are then conducted for concentric tube heat exchanger with different twisted tape configuration for optimal design. The results show that the enhancement in the rate of heat transfer in annularly corrugated tube heat exchanger with twisted tape is about 235.3% and 67.26% when compared with the plain tube and annularly corrugated tube heat exchangers without twisted tapes respectively. Based on optimal results, for a 120 ekW diesel generator, the application of corrugated tube with twisted tape concentric tube heat exchanger can save 2250 gal of fuel, $11,330 of fuel cost annually and expected payback of 1 month. In addition, saving in heating fuel also reduces in CO₂ emission by 23 metric tons a year.     

Optimum tip gap and orientation of multi-piezofan for heat transfer enhancement of finned heat sink in microelectronic cooling

Piezoelectric fans can be manipulated to generate airflow for cooling microelectronic devices. Their outstanding features include noise-free operation, low power consumption and suitability for confined spaces. This paper presents experimental optimization of tip gap and orientation angle of three piezoelectric fans (multi-piezofan) to maximize the heat removal performance of finned heat sink for microelectronic cooling. Design of experiments (DOE) approach is used for the optimization, and a three dimensional simulation using FLUENT 6.3.2 is carried out to better understand the flow induced by the multi-piezofan and the resulting heat transfer from the heat sink surface. For the optimization, the Central Composite Design (CCD) of response surface methodology (RSM) is exploited from the Design Expert software. In the numerical model, the flow induced by the piezofan is treated as incompressible and turbulent; the turbulence is taken care by the shear stress transport (SST) k–ω model. The experimental results are found to be in good agreement with the predictions. Out of 13 experimental trials determined by CCD, the optimum tip gap and fan orientation are found to be δ = 0.17 and 90° respectively. At this condition, an enhancement in convective heat transfer coefficient exceeding 88% is achieved, compared to natural convection.     

Multi-parameters optimization for microchannel heat sink using inverse problem method

This work describes an inverse problem method to optimize the geometric design for microchannel heat sinks using a novel multi-parameter optimization approach, which integrates the simplified conjugate-gradient scheme and a fully developing three-dimensional heat transfer and flow model. Overall thermal resistance is the objective function to be minimized with number of channels, N, channel aspect ratio, α, and the ratio of channel width to pitch, β, as search variables. With a constant bottom area (10 mm × 10 mm), constant heat flux applied to the heat sink bottom surface (100 W cm^?2), and constant pumping power (0.05 W), the optimal design values are N = 71, α = 8.24, and β = 0.6, with a minimum overall thermal resistance of 0.144 K W^?1. Increasing pumping power reduces overall thermal resistance of the optimal design; however, the design’s effectiveness declines significantly under high pumping power. The N and α values in the optimal design increase and β decreases as pumping power increases.     

The application of the CFD and Kriging method to an optimization of heat sink

The shape optimization of the plate-fin type heat sink with an air deflector is numerically performed to minimize the pressure loss subjected to the desired maximum temperature and geometrical constraints. A function evaluation using the FVM, in general, is required much computational costs in fluid/thermal systems. Thus, global approximate optimization techniques have been introduced into the optimization of fluid/thermal systems. In this study, the Kriging method, which is one of the metamodels, associated with the computational fluid dynamics (CFD) is used to obtain the optimal solutions. The Kriging method can dramatically reduce a computational cost by 1/6 times compared to that of the SQP method so that its efficiency can be validated. The results also show that when the temperature rise is less than 40 K, the optimal design variables are B₁ = 2.44 mm, B₂ = 2.09 mm, and t = 7.58 mm.     

Fluid flow and heat transfer characteristics of cross-cut heat sinks

In this study, effects of cross-cuts on the thermal performance of heat sinks under the parallel flow condition are experimentally studied. To find effects of the length, position, and number of cross-cuts, heat sinks with one or several cross-cuts ranging from 0.5 mm to 10 mm were fabricated. The pressure drop and the thermal resistance of the heat sinks are obtained in the range of 0.01 W<P_p < 1 W. Experimental results show that among the many cross-cut design parameters, the cross-cut length has the most significant influence on the thermal performance of heat sinks. The results also show that heat sinks with a cross-cut are superior to heat sinks containing several cross-cuts in the thermal performance. Based on experimental results, the friction factor and Nusselt number correlations for heat sinks with a cross-cut are suggested. Using the proposed correlations, thermal performances of cross-cut heat sinks are compared to those of optimized plate-fin and square pin-fin heat sinks under the constant pumping power condition. This comparison yields a contour map that suggests an optimum type of heat sink under the constraint of the fixed pumping power and fixed heat sink volume. The contour map shows that an optimized cross-cut heat sink outperforms optimized plate-fin and square pin-fin heat sinks when 0.04 < log L1 < 1.     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

Modelling of an adsorption system driven by engine waste heat for truck cabin A/C. Performance estimation for a standard driving cycle

This paper presents the main characteristics of an innovative cooling system for the air conditioning of a truck cabin, as well as a first estimation of its performance during a standard driving cycle, obtained with a specifically developed vehicle-engine-cooling system overall model. The innovative cooling system consists of a water–zeolite adsorption–desorption system, which employs the waste heat from the engine to produce the cooling of the vehicle cabin. The developed global model is completely dynamic and is able to: reproduce the operation of the engine through a standard driving cycle, evaluate the waste heat available at the engine hydraulic loop; calculate the sequential operation of an adsorption–desorption system, calculate the condensed water per cycle, the cooling effect produced at the evaporator, and finally, the temperature and humidity evolution of the air inside the cabin. The model was validated by experimental data. The experimental tests were performed in a lab-scale adsorption chiller prototype specifically designed and realized to be driven by the low grade waste heat (80–90 °C) from the engine coolant loop of a truck. The experimental activity carried out showed that the chiller is able to generate up to 5 kW of peak cooling power at 10 °C (35 °C of condensation temperature) with a COP of 0.6. The obtained results show that the system could be able to provide a significant amount of the required cooling.     

Investigation of natural convection induced outer side heat transfer rate of coiled-tube heat exchangers

Natural convection induced heat transfer has been studied over the outer surface of helically coiled-tube heat exchangers. Several different geometrical configurations (curvature ratio δ ε [0.035, 0.082]) and a wide range of flow parameters (60 <= T_tank <= 90, T_in = 19 and 60 <= T_in <= 90, T_tank = 20, 4000 <= Re <= 45000) have been examined to broaden the validity of the results gained from this research. A fluid-to-fluid boundary condition has been applied in the numerical calculations to create the most realistic flow configurations. Validity of the numerical calculations has been tested by experiments available in the open literature. Calculated results of the inner side heat transfer rate have also been compared to existing empirical formulas and experimental results to test the validity of the numerical computation in an independent way from the outer side validation of common helical tube heat exchangers. Water has been chosen to the working fluid inside and outside of the coiled tube (3 < Pr < 7). Outer side heat transfer rate along the helical tube axis has been investigated to get information about the performance of the heat transport process at different location of the helical tube. It was found that the outer side heat transfer rate is slightly dependent on the inner flow rate of any helical tube in case of increasing temperature differences between the tank working fluid temperature and the coil inlet temperature. A stable thermal boundary layer has been found along the axial direction of the tube.In addition to this the qualitative behavior of the peripherally averaged Nusselt number versus the axial location along the helical tube function is strongly dependent on the direction of the heat flow (from the tube to the storage tank and the reversed direction). Inner side heat transfer rate of helical coils have also been investigated in case of fluid-to-fluid boundary conditions and the calculation results have been compared with different prediction formulas published in the last couples of decades.     

Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers

Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solution sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems.In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capacity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side.Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated.In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U_overall varied from 970 W/m² °C (170 Btu/h ft² °F) to 2270 W/m² °C (400 Btu/h ft² °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry.     

Performance of low-temperature differential Stirling engines

In this paper, two single-acting, twin power piston and four power pistons, gamma-configuration, low-temperature differential Stirling engine are designed and constructed. The engine performance is tested with air at atmospheric pressure by using a gas burner as a heat source. The engine is tested with various heat inputs. Variations of engine torque, shaft power and brake thermal efficiency at various heat inputs with engine speed and engine performance are presented. The Beale number obtained from testing of the engines is also investigated. The results indicate that, for twin power piston engine, at a maximum actual heat input of 2355 J/s with a heater temperature of 589 K, the engine produces a maximum torque of 1.222 N m at 67.7 rpm, a maximum shaft power of 11.8 W at 133 rpm, and a maximum brake thermal efficiency of 0.494% at 133 rpm, approximately. For the four power pistons engine, the results indicate that at the maximum actual heat input of 4041 J/s with the heater temperature of 771 K, the engine produces a maximum torque of 10.55 N m at 28.5 rpm, a maximum shaft power of 32.7 W at 42.1 rpm, and a maximum brake thermal efficiency of 0.809% at 42.1 rpm, approximately.     

Effect of fin pitches on the optimum heat transfer performance of crimped spiral fin-and-tube heat exchangers

This study conducted experiments on the optimized fin pitch for crimped spiral fin-and-tube heat exchangers. The experiments covered a size range of 2.4–6.5 mm, which is the manufacturing limitation for this kind of fin. The water-flow arrangement used in this experiment combined the parallel cross-flow and the counter cross-flow in a two-row configuration. Ambient air was used as the working fluid on the air-side, and hot water was used on the tube-side. The effects of fin pitches on the heat transfer coefficient and pressure drop characteristics were studied. The results clearly showed that the convective heat transfer coefficient (h_o) for a fin pitch of 2.4 mm is relatively low compared with that of other fin pitches with the same air frontal velocity. Using larger fin pitches (i.e., 4.2, 6.2, and 6.5 mm) resulted in negligible differences in the pressure drop. Moreover, this work introduces the parameter of three performances indexes, which can be expressed as the ratio of the desired output to the required input, for optimization purposes. Due to the difference in optimum fin pitch obtained by these performance indexes, an intersection analysis was conducted. The results indicated that the optimum fin pitch is 4.2 mm for this work, which could be valuable for the effective design for industrial thermal-system applications.     

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.     

Optimum thermal design of triangular,trapezoidal and rectangular grooved microchannel heat sinks

A three-dimensional numerical simulation is conducted to investigate the effect of geometrical parameters on laminar water flow and forced convection heat transfer characteristics in grooved microchannel heat sink (GMCHS). Four geometry variables which are; the depth, tip length, pitch and orientation of the cavities are taken into account in order to optimize the aluminum heat sink design. These geometric parameters could change the cavity shape from triangular to trapezoidal and then to rectangular shape. The governing and energy equations are solved using the finite volume method (FVM). The performance of GMCHS is evaluated in terms of Nusselt number ratio, thermal/hydraulic performance (JF) and isotherm and streamlines contours. The results showed that the trapezoidal groove with groove tip length ratio of δ = 0.5, groove depth ratio β = 0.4, groove pitch ratio of ψ = 3.334, grooves orientation ratio of ζ = 0.00 and Re = 100 is the optimum thermal design for GMCHS with Nusselt number enhancement of 51.59% and friction factor improvement of 2.35%.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Optimization of heat flow analysis for exceeding hundred watts in HI-LEDs projectors

This study conducted a heat flow-lighting performance test to evaluate and analyze an exceeding hundred watts of HI-LEDs (higher-power LEDs) projector. In addition, a Windows-based optimization program was developed to optimize the heat sink of the HI-LEDs projector. The results indicated that the maximal LED temperature of the projector was 108 °C at a steady temperature state (approximately 1 h of operation). When the metal-core printed circuit board (MCPCB) of the projector was replaced with a vapor-chamber PCB (VCPCB), the LED temperature decreased to 87.6 °C. Iterative calculations were subsequently performed using the self-developed Windows-based optimization program for determining the optimal fin interval (8.94 mm) and thickness (2.56 mm). These optimal parameters were simulated using CFD (computational fluid dynamics), and the results were compared with those of the original 150-W HI-LEDs projector. A Rayleigh number of 6.1304 × 10⁵ was obtained, indicating a laminar flow model. When the optimized heat sink was used, the LED temperatures for the MCPCB and VCPCB HI-LEDs projector samples decreased by 14 and 9.74 °C, respectively. The h value of the optimized HI-LEDs projector (8.094 W/m²k) was higher than that of the original HI-LEDs projector (7.154 W/m²k), and its weight was reduced by approximately 2%.     

Optimal geometry of L and C-shaped channels for maximum heat transfer rate in natural convection

The present paper documents the geometric optimization of L and C-shaped channels in laminar natural convection subject to global constraints. The objective is to maximize the heat transfer rate from the hot wall to the coolant fluid. Three different configurations were considered: (i) an L-shaped asymmetric vertical heated channel with an adiabatic horizontal inlet, (ii) an asymmetric vertical heated channel with an adiabatic vertical outlet, and finally, (iii) a C-shaped vertical channel with horizontal inlet and outlet. The two first configurations are free to morph according to two degrees of freedom: the wall-to-wall spacing and inlet (or outlet) height. The third configuration is optimized with respect to the wall-to-wall spacing, and the heights of the inlet and outlet ports. The effect of the inlet or outlet horizontal adiabatic duct lengths is also investigated. The optimization is performed numerically by using the finite element technique, in the range 10⁵ < Ra < 10⁷ for Pr = 0.7, where Ra is the Rayleigh number based on a fixed total height H of the channel. The numerical results show that optimization is relevant, since the three degrees of freedom considered have a strong effect on the heat transfer delivered from the hot wall to the fluid. The optimal geometric characteristics obtained numerically (i.e., optimal spacing, optimal height and lengths) are reported and correlated within a 7.5% maximal disagreement range.     

Forced convection heat transfer from an elliptical cylinder to power-law fluids

Forced convection heat transfer to incompressible power-law fluids from a heated elliptical cylinder in the steady, laminar cross-flow regime has been studied numerically. In particular, the effects of the power-law index (0.2 ? n ? 1.8), Reynolds number (0.01 ? Re ? 40), Prandtl number (1 ? Pr ? 100) and the aspect ratio of the elliptic cylinder (0.2 ? E ? 5) on the average Nusselt number (Nu) have been studied. The average Nusselt number for an elliptic cylinder shows a dependence on the Reynolds and Prandtl numbers and power-law index, which is qualitatively similar to that for a circular cylinder. Thus, heat transfer is facilitated by the shear-thinning tendency of the fluid, while it is generally impeded in shear-thickening fluids. The average Nusselt number values have also been interpreted in terms of the usual Colburn heat transfer factor (j). The functional dependence of the average Nusselt number on the dimensionless parameters (Re, n, Pr, E) has been presented by empirically fitting the numerical results for their easy use in process design calculations.     

Glass tempering heat transfer coefficient evaluation and air jets parameter optimization

A design of experiments (DOE) matrix of CFD simulations is used to create a mathematical model able to calculate the heat transfer coefficient for nine circular confined air jets vertically impinging on a flat plate. Typical air jets dimensions and process parameter values used in glass tempering are evaluated. The flat plate temperature is set to the constant value of 640 °C. Two different values of jet diameter (4 mm, 8 mm), of air velocity at nozzle exit (110 m/s, 140 m/s), of jet-to-jet spacing (40 mm, 60 mm), of jet-to-plate distance (40 mm, 60 mm) and of nozzle height (20 mm, 60 mm) are considered. Implemented into a Visual Basic application, the mathematical model found allow the instant evaluation of heat transfer parameters and to optimize air jets parameter configuration.     

Visualization investigation of the flow and heat transfer in thermoacoustic engine driven by loudspeaker

Heat transfer process in thermoacoustic engine is affected by acoustic oscillation which makes it different from the heat transfer in steady flow. This study pays attention to the flow and heat transfer characteristics of thermoacoustic engine driven by loudspeaker. Thermal infrared imager and particle image velocimetry (PIV) were used to investigate the temperature and flow fields under two heat levels (150 °C and 200 °C). The radial and axial temperature distribution was analyzed through dimensionless temperature. To explore the appropriate working frequency, resonance characteristic was discussed. The experimental results illustrated that the first resonance frequency is the most effective driving frequency where thermoacoustic system shows the best performance. Heat transfer mode changed from natural convection to forced convection with the addition of acoustic oscillation. Original temperature field induced by heat convection was destroyed and temperature gradient redistributed as parabolic after sound addition.