Analysing the efficiency of thermopressor application in the charge air cooling system of combustion engine

As the analysis of the research results has shown, the use of a thermopressor makes it possible to increase the fuel and energy efficiency of a ship power plant in a wide range of the operation parameters. It can be achieved by cooling the charge air before the engine inlet receiver and by reducing the compression work of the turbocharger. A scheme with the thermopressor application in the cooling system of a low-speed main engine and in the system for utilizing the exhaust gases heat in a heat recovery boiler of one and two pressures was proposed. The use of thermopressors led to a decrease in the compressor power consumption, and therefore in the turbine required power. Therefore, it was appropriate to pass (bypass) the excess amount of gas past the turbine. Accordingly, the thermal potential of exhaust gases was increased. As a result, the temperature of gases at the inlet to the heat recovery boiler was increased by 10-15 °C, and gases heat was increased by 10-15% respectively. The obtained additional steam is advisable to use for driving the utilization turbine generator, thereby reducing the load on the ship's power plant, with a corresponding decrease in fuel consumption of diesel generators by 2-4%.


Introduction
Increasing the power of ship power plants based on internal combustion engines was carried out by increasing the flow rate of the working fluid (charge air) and the useful work of combustion products expanding. At the same time, there was a simultaneous reduction in the work for air compressing, which was achieved by cooling air and deep utilization of the combustion products energy. That, in turn, led to a decrease in specific fuel consumption and an increase in efficiency.

Literature review
Low-, medium-and high-speed internal combustion engines are widely used in power plants of transport and stationary energy. The air parameters at the turbocharger discharge and at the inlet to the cylinders significantly affect the fuel and energy efficiency of the engine. Every 10 degrees an increase in air temperature causes a decrease in the effective efficiency е of marine lowspeed diesel engines by 0.5-0.7% [1,2] with a corresponding increase in specific fuel consumption gе [3]. The decrease in engine power is caused by a decrease in the mass air flow in the cylinders due to a decrease in the air density with an increase in temperature [4,5].
The development of modern energy efficient technologies for power plants can cause a low level of waste heat use [6,7]: the temperature of gases after the heat recovery boilers is about 180 °C; charge air * Corresponding author: dimitriyko79@gmail.com temperature is 140-220 °С; the temperature of the engine cylinders cooling water is 90-120 °C. Heat with such a thermal potential can be used in several ways: 1. The use of the combustion products heat for the steam production in the heat recovery boiler (reduction of the specific fuel consumption gе is 2-3%) [8,9].
3. To improve the turbocharge system [12,13]. 4. The use of the high-temperature cooling method of the internal combustion engine [14].
5. The use of combined energy production technologies with simultaneous production of heat (steam, hot water), electrical energy (generator, shaft generator) and cold (heat-using ejector and absorption refrigeration machines) [15].
The most common way to improve the fuel and energy efficiency of a power plant is contact cooling of the air flow by water injection [16,17]. It is promising to cool the charge air of the internal combustion engine with a thermopressor which provides an increase in efficiency and a reduction in fuel consumption due to a decrease in temperature and an increase in compressed air pressure. It leads, in turn, to a decrease in the compression power consumption [18]. Changing operating conditions of power plants, and, consequently, the thermal load on the thermopressor, require rational organization of work processes and the determination of a rational design pressure increase that would provide the maximum effect. Thermopressor technologies are based on the implementation of the thermogasdynamic compression effect in special contact heat exchangers (aerothermopressors or thermopressors), which consists in increasing the pressure while decreasing the temperature during the evaporation of a finely dispersed liquid injected into a gas flow moving at a speed close to sound [19,20].
On the basis of the presented analysis, on the current state of development of technologies for increasing the fuel and energy efficiency of power plants based on internal combustion engines, the main task of the research was formulated: to develop and analyze improved schemes of thermopressor systems for utilizing the energy of combustion products with cooling the working fluid (air) of the engines, principles and methods of their implementation.

Research methodology
A comparative-calculation method of research was used in this work to determine the thermodynamic and energy efficiency of a thermopressor cooling system as part of a power plant. A proprietary software package was developed to use this method and it allows: to simulate working processes in a thermopressor; to calculate the main structural elements of the thermopressor; to calculate the energy efficiency and main indicators of the engine when using thermopressor cooling systems, taking into account changes in climatic and hydrometeorological conditions, as well as particular operating modes of the power plant.
The calculation of the thermopressor characteristics (air pressure Pair, air temperature Tair, air velocity wair, water velocity ww, etc.) and its basic geometric parameters of the flow path under specified operating conditions of the power plant engine was carried out according to the indicated methods [21][22][23]. The calculation took into account the pressure loss in the nozzle (confuser), working chamber, diffuser [24,25], as well as with the frontal resistance of injected water droplets [26,27].
The calculation of the main parameters of the internal combustion engine was carried out using the software packages Diesel-RK and CEAS (MAN B&W) [28] taking into account the change in the air parameters at the inlet to the engine cylinders and partial operating modes. To use these software systems, an additional program was developed in which the parameters of wet gas (air) were determined at the inlet and outlet of the turbocharger.

Results
A scheme in which the thermopressor is used as a charge air cooler behind a turbocharger of low-speed internal combustion engines with a waste gas heat recovery system in a waste heat recovery boiler is shown in Fig.  1a. The analysis was carried out in relation to the standard traditional scheme for charge air cooling of ship internal combustion engines. The calculations of a low-speed main engine 5S50MC-C (MAN B&W) with a power of Nе = 8300 kW and a speed of n = 105 rpm were carried out. The calculation of the thermopressor parameters was carried out taking into account its joint work with a turbocharger of the pressure-charging system. The increase in pressure in the outlet thermopressor ΔРatp significantly depends on the value of the temperature decrease during cooling Δtatp, and therefore the air temperature at the inlet tatp1 is very important. The temperature in front of the thermopressor corresponds to the discharge air temperature of the turbocharger. In the scheme with one-stage compression, the temperature in front of the thermopressor is tc2 = 190-270 °С (Fig. 2a). The temperature behind the turbocharger is the higher, the higher the inlet temperature and the degree of pressure increase in the turbocharger πc. Since the thermopressor cooling system, in fact, is a two-stage air compression (the first stage is a turbocharger, the second stage is a thermopressor), the compression ratio πc of such a scheme will be lower (Fig. 2b), and therefore tc2 = tc2' = 170-250 °С, that is, the temperature in front of the thermopressor decreases, and accordingly the compression ratio in the air thermopressor πatp will also be lower. Since the required compression ratio for the turbocharger πc decreases, the compressor work for compression lc decreases accordingly. For a system with total πc.atp = 4.25 the decrease in work is Δlc = 10-13 kJ/kg; with total πc.atp = 4.00 -Δlc = 9.0-12.5 kJ/kg; with total πc.atp = 3.85 -Δlc = 8.5-12.0 kJ/kg (Fig. 3). Reducing the work on compression lc allows to reduce the compressor power (Fig. 2b) by ΔNc = 100-200 kW (10.0-11.5 %), with the same air consumption in the engine.
The minimum temperature at the outlet of the thermopressor tatp2 was taken to be a temperature 2-3 °C higher than the dew point temperature. It was taken into account that water injection continues in the thermopressor until the air is completely saturated, that is, φ = 100%. The decrease in the air temperature is Δtatp = 110-160 °С, which makes it possible to increase the air pressure after the turbocharger by ΔР'atp = 520-670 kPa (Fig. 3a) with "ideal" compression (without taking into account friction losses about the channel wall), and with real compression in the thermopressor air pressure increase is ΔРatp = 340-480 kPa, this corresponds to ΔР'atp = 15-18 % and ΔР'atp = 10-13% (Fig. 3b).
At the same time, the degree of decrease in air temperature in the thermopressor was Tatp1/Tatp2 = 1.30-1.45. The thermopressor is the second stage of compression in the pressure-charging system; accordingly, it is appropriate to evaluate its efficiency by the degree of pressure increase πatp = 1.10-1.13. A decrease in air temperature and an increase in pressure in the thermopressor are quite significant, which affects the decrease in flow rates in the turbocharger, as a result, the exhaust gases temperature of the utilization turbine (UT) increases (Fig. 4a). Thus, at a constant temperature of gases at the UT inlet (tg1 = 300-400 °C), the temperature of the exhaust gases is tg2 = 205-285 °C, in the scheme by using the thermopressor the temperature of the exhaust gases is tg2 = 220-300 °C, which is 15 °C higher for the basic version.
Comparison of the thermopressor with the heat exchanger, from the point of view of the heat exchanger (charge air cooler), shows that the heat load on the thermopressor is Qatp = 1600-3700 kW (Fig. 4b). However, the air temperature at the discharge of the thermopressor is still high and amounts to tatp = 65-85 °е. Therefore, it is advisable to install an additional heat exchanger behind the thermopressor. The heat load on such charge air cooler is Qcac2 = 200-1200 kW, hence the total heat load on the thermopressor and the additional heat exchanger is Qt2 = 2000-4800 kW, which is less than for the standard charge air cooling system Qst2 = 2400-5200 kW (7-15 %). The increase in the thermal load on the standard charge air cooler can be explained by reducing the air temperature in the turbocharger with the thermopressor at 20-25 °C.
The efficiency of thermopressor operation was estimated taking into account the energy consumption for the supply of fresh water for injection in the thermopressor nozzle. According to calculations, the water consumption for injection into the thermopressor is Gw = 0.6-1.2 kg/s, while the pump drive power is Nw = 0.3-0.7 kW. The moisture content of the air increases by Δdatp = 40-60 g/kg. The injection pump power is rather small (0.3-0.5%) compared to the decrease in the compressor power ΔNc = 100-200 kW.
The use of the thermopressor leads to a decrease in the turbocharger power, and then the unnecessary heat drop (work) of the TC turbine decreases, the required power of the turbine and the required gas flow rate decrease. Hence, it is appropriate to pass the excess amount of gas past the turbine, due to which the exhaust gases temperature in front of the heat recovery boiler increases. Hence, the thermal potential of waste gases increases, which can be used in a waste heat recovery boiler. So the temperature at the heat recovery boiler inlet grows by almost Δtrb = 10-15 °С, taking into account the temperature of the gases at the heat recovery boiler outlet is constant and equal to trb2 = 160 ° С, the additional heat load (with the corresponding additional consumption steam) is ΔQrb = 150-300 kW (10-15%).
As shown by the calculations of the thermal scheme, the amount of steam generated in the heat recovery boiler is more than enough to meet the needs of this dry cargo ship. So, it is advisable to use the obtained additional steam to drive a utilization turbine generator (UTG), thereby reducing the load on the ship's power plant, with a corresponding decrease in fuel consumption for diesel generators.
A scheme of the thermopressor charge air cooling system with integrated heat recovery from exhaust gases in the heat recovery boiler with two pressure circuits and UTG is shown in Fig. 1b. Analysis of the UTG operation shows (Fig. 5) that an increase in heat power in a waste heat recovery boiler of two pressures allows an increase in steam production by ΔDutg = 0.07-0.12 kg/s (250-430 kg/h), which increases the power of UTG by ΔNutg = 45-90 kW (6-10% of the rated power of the UTG). When two MAN B&W 7L16/24 diesel generators are installed on board with a rated power Ne = 770 kW and a specific fuel consumption ge = 195 g/(kW•h), a decrease in the load on the ship's power plant when the UTG is turned on (Fig. 5) reduces the specific fuel consumption by Δge = 4-7 g/(kW•h), with a temporary reduction in fuel consumption ΔGst = 10-18 kg/h (240-430 kg/day). The analysis of economic feasibility shows that the reduction in fuel consumption per voyage within 28 days will be ΔGv = 7-12 tons. Taking into account the summer operation of the vessel, the reduction in fuel consumption per year will be ΔGyear = 90-150 tons. The analysis of circuit solutions to use the thermopressor technologies for charge air cooling of engines is studied. It showed that the load reduction on the ship power plant with a corresponding reduction in the fuel consumption of diesel generators is 2-4%. In this case, the production of an additional amount of steam by 10-15% can be ensured. The branches of the predominant application of thermopressor technologies include plants of stationary and marine power energy based on internal combustion engines and gas turbine engines. The use of thermopressor technologies provides a significant reduction in fuel consumption and can contribute to an increase in the energy security level of the state.