Christelle Périlhon

Association of Finite-Time Thermodynamics and a Bond-Graph Approach for Modeling an Endoreversible Heat Engine

In recent decades, the approach known as Finite-Time Thermodynamics has provided a fruitful theoretical framework for the optimization of heat engines operating between a heat source (at temperature ) and a heat sink (at temperature ). The aim of this paper is to propose a more complete approach based on the association of Finite-Time Thermodynamics and the Bond-Graph approach for modeling endoreversible heat engines. This approach makes it possible for example to find in a simple way the characteristics of the optimal operating point at which the maximum mechanical power of the endoreversible heat engine is obtained with entropy flow rate as control variable. Furthermore it provides the analytical expressions of the optimal operating point of an irreversible heat engine where the energy conversion is accompanied by irreversibilities related to internal heat transfer and heat dissipation phenomena. This original approach, applied to an analysis of the performance of a thermoelectric generator, will be the object of a future publication.

Numerical study of the waste heat recovery potential of the exhaust gases from a tractor engine

The paper presents an analysis of the possibilities of exhaust gas heat recovery for a tractor engine with an output power of 110 kW. On the basis of a literature review, the Rankine cycle seems to be the most effective way to recover the exhaust gas energy. This approach reduces the fuel consumption and allows engines to meet future restrictions on carbon dioxide emissions. A simulation model of the engine by means of a one-dimensional approach and a zero-dimensional approach was built into the simulation code AVL BOOST, and a model of the Rankine cycle was implemented. The experimental values of the effective power of the engine, the mass flow and the exhaust gas temperature were used to validate the engine model. The energy balance of the engine shows that more than 28.9% of the fuel energy is rejected by exhaust gases. Using the engine model, the energy and the exergy of the exhaust gases were studied. An experimental study of the real working cycle of a tractor engine revealed that the engine operates most of the time at a constant speed (n = 1650 r/min) and a constant load (brake mean effective pressure, 10 bar). Finally, Rankine cycle simulations with four working fluids were carried out at the most typical operating point of the engine. The simulation results reveal that the output power of the engine and the efficiency of the engine increase within the range 3.9–7.5%. The highest value was achieved with water as the working fluid while the lowest value was obtained with the organic fluid R134a. The power obtained with water as the working fluid was 6.69 kW, which corresponds to a Rankine cycle efficiency of 15.8%. The results show good prospects for further development of the Rankine cycle.

Thermodynamic Modelling of an Ejector with Compressible Flow by a One-Dimensional Approach

The purpose of this study is the dimensioning of the cylindrical mixing chamber of a compressible fluid ejector used in particular in sugar refineries for degraded vapor re‑compression at the calandria exit, during the evaporation phase. The method used, known as the “integral” or “thermodynamic model”, is based on the model of the one‑dimensional isentropic flow of perfect gases with the addition of a model of losses. Characteristic curves and envelope curves are plotted. The latter are an interesting tool from which the characteristic dimensions of the ejector can be rapidly obtained for preliminary dimensioning (for an initial contact with a customer for example). These ejectors, which were specifically designed for the process rather than selected from a catalog of standard devices, will promote energy saving.

Influence of Heat Transfer on Gas Turbine Performance

In the current economic and environmental context dominated by the energy crisis and global warming due to the CO2 emissions produced by industry and road transportation, there is an urgent need to optimize the operation of thermal turbomachinery in general and of gas turbines in particular. This requires exact knowledge of their typical performance. The performance of gas turbines is usually calculated by assuming an adiabatic flow, and hence neglecting heat transfer. While this assumption is not accurate for high turbine inlet temperatures (above 800 K), it provides satisfactory results at the operating point of conventional machines because the amount of heat transferred is generally low (less than 0.5% of thermal energy available at the turbine inlet). Internal and external heat transfer are therefore neglected and their influence is not taken into account. However, current heating needs and the decentralized production of electrical energy involve micro Combined Heat and Power (CHP) using micro-gas turbines (20-250 kW). In aeronautics, the need for a power source with a high energy density also contributes to interest in the design of ultra-micro gas turbines. These ultra and micro machines, which operate on the same thermodynamic principles as large gas turbines, cannot be studied with the traditional adiabatic assumption, as has been underlined by many authors such as Ribaud (2004), Moreno (2006) and Verstraete et al. (2007). During operation, heat is transferred from the turbine to the outside, bearing oil, casing and compressor, thus heating the compressor and leading to a drop in turbine performance. Consequently, the performances reported on the maps developed under the adiabatic assumption are no longer accurate.

Optimization of automotive Rankine cycle waste heat recovery under various engine operating condition

An optimization study of the Rankine cycle as a function of diesel engine operating mode is presented. The Rankine cycle here, is studied as a waste heat recovery system which uses the engine exhaust gases as heat source. The engine exhaust gases parameters (temperature, mass flow and composition) were defined by means of numerical simulation in advanced simulation software AVL Boost. Previously, the engine simulation model was validated and the Vibe function parameters were defined as a function of engine load. The Rankine cycle output power and efficiency was numerically estimated by means of a simulation code in Python(x,y). This code includes discretized heat exchanger model and simplified model of the pump and the expander based on their isentropic efficiency. The Rankine cycle simulation revealed the optimum value of working fluid mass flow and evaporation pressure according to the heat source. Thus, the optimal Rankine cycle performance was obtained over the engine operating map.

A Study of Waste Heat Recovery Impact on a Passenger Car Fuel Consumption in New European Driving Cycle

In this article the effect of waste heat recovery (WHR) system by means of Organic Rankine cycle (ORC) on passenger car engine fuel consumption was studied. A vehicle driving model was developed in order to determine the engine operating points in New European Driving Cycle (NEDC). In order to evaluate exhaust gases enthalpy and fuel consumption at engine operating points corresponding to the driving cycle the engine was experimentally tested in steady mode. The exhaust gases temperature was measured at a location situated 1.5 m downstream the exhaust valves considering that place as the inlet of ORC heat exchanger. A simulation model of ORC was developed using R245fa as a working fluid. Numerical results revealed that maximum recovered power was 1.69 kW. The contribution of the WHR system on the vehicle fuel consumption was assessed by reduction in cumulated fuel consumption for a NEDC. Applying an ORC to engine exhaust gases reduces cumulated fuel in a NEDC test from 0.441 to 0.414 kg. In relative values this reduction accounts to 5.9 %.

Design and experimental investigation of adryer with two types of combined solar concentratorsusing two natural heattransfer fluids

In this experimental study, a new solar dryer has been designed and manufactured by combining a concave solar concentrator with a series of convergent lenses whose concentrated radiation are reflected on the same absorber. Our main goal for this combination is to reduce the thermal losses by increasing the receiver bottom temperature. In our dryer, the receiver design uses air and water as the heat transfer fluid; these two fluids are heated and sent a heat exchanger to raise the drying air temperature. Furthermore, in order to improve the thermal performance, our approach is based on some techniques such as; the combination of two types of solar concentrators, simultaneous use of two natural heat transfer fluids, tilting concentrators, the insertion of obstacles at the absorber, and a specific solar tracker for explosive atmosphere. Our dryer has been tested for the first time in gas filling plant in Morocco for drying painted gas cylinders. The experimental results show that compared with flat-plate solar collector, the proposed dryer can significantly improve the performance in terms of air drying temperature. In fact, our solar dryer has reduced the drying time from 420 seconds to just 40 seconds and has improved drying air the temperature from 40 °C to 65 °C. Also, the health risks of workers have been reduced and the number of painted bottles has been increased by more than 43%.

Study on the combustion process in a modern diesel engine controlled by pre-injection strategy

The paper aims to study the combustion process in a modern diesel engine over the engine operating map. In order to study the rate of heat release (ROHR), an automotive diesel engine was experimentally tested using the injection parameters factory defined. The experimental test was conducted over the engine operating map as the engine speed was limited to 2400 rpm. Then, an engine simulation model was developed in AVL Boost. By means of that model the ROHR was estimated and approximated by means of double Vibe function. In all engine operating points we found two peaks at the ROHR. The first is a result of the pilot injection as the second corresponds to the main injection. There was not found an overlap between both peaks. It was found that the first peak of ROHR occurs closely before top dead center (BTDC) at partial load than full load. The ROHR peak as a result of main injection begins from 4°BTDC to 18°ATDC. It starts earlier with increasing engine speed and load. The combustion duration varies from 30 oCA to 70 °CA. In order to verify the results pressure curve was estimated by means of defined Vibe function parameters and combustion duration. As a result, we observed small deviation between measured and simulated pressure curves.