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Design and fabrication of the heat exchanger by smooth tube for recovering, utilizing the exhaust gas energy of small diesel engine aiming at heating bio-oils.

INTRODUCTION

In among of main power in the transportation, construction, fishery and agriculture machinery, engine has played an important part and consumed more than 60% of fossil fuel, thus it is able to result in exhausting the fossil fuel. Recent propensity about using the energy sources aiming at reducing the fossil fuel consumption as well as pollution is considered as urgent task. Up to now, the major consumer of fossil fuel is the internal combustion engines (ICE), however only about 30-40% energy of combustion in the engine chamber is transformed into useful mechanical work. Besides, the rest heat source expelled to the environment or lost through exhaust gases and cooling water/oil are approximately 25-35%, therefore it results in polluting the environment seriously, hence it is necessary to utilize and recover the waste heat to increase the heat efficiency of internal combustion engines. The waste heat recovery and utilization not only saves energy but also reduces the toxic pollution. Engine manufacturers have implemented and improved the latest techniques to increase thermal efficiency by enhancing the fuel-air mixing, using turbo-charger, and variable valve timing or advance combustion chamber.

Many researchers have recognized that WH utilizing and recovery from engine exhaust gas was considered as the potential in order to decrease fuel consumption without increasing emissions. In the systems of WH, the energy may be recovered by many different technologies. Many researchers about recovering the WH both on diesel engines [7] and gasoline engines [6,17] have been carried out. The results of combination of ETC (Electric turbo compound) and TEG (Thermo-electric generator) led 3% to 5% of fuel saving and 1% to 4% of CO2 reduction [8] as this combination system was installed in was noticed. A combination between a TEG and an ORC (Organic Rankine cycle) system aiming at recovering the WR energy was investigated by Zhang et al. [30]. Alberto. A.B et al. [2] showed the way of recovering the WH from exhaust gases of ICE by using Organic Rankine Cycle (ORC) system. In this case, the engine power increased in comparison with 3.4% of the flow rate of fuel energy on average. However, the average flow rate of fuel energy increased up to 5.1% with 8.2% of top improvements if combined two ORCs. Moreover, the WH recovery by using Stirling engine was studied by Douglas C.G. [10], this study showed that, Stirling cycle is useful, efficient to recover the WH of ICE without intake or exhaust gas. Similar results about Stirling engine advantages operated by WH such as high efficiency, favorable and quiet operation, non-emissions, low maintenance and vibrations, usability with many different fuels was presented by Wail. A et al. [29].

Pandiyarajan. V et al. [22] showed the way of recovering WH from ICE by tube heat exchanger. According to this method, the capsules of phase change material were used for heat storage. The efficiency of recovering the heat was nearly 99% at all load, about 10-15% of total WH was recovered. Besides, the significant reduction of the fuel consumption of engine and toxic emissions by recovering WH was shown by Janak. R et al. [18]. Manzela et al. [19] studied about the recovering the WH from automobile engine for the absorption refrigeration based on the working fluid of ammonia and water. The structure of refrigeration system by recovering WH, the calculation of WH recovery rate was also shown by Hilalil et al. [13].Moreover, there are many methods for designing the heat exchanger, they are included double or smooth tube, tube, plate, plate-fin, spiral heat or frame exchangers [28].

About calculation and design of heat exchangers, Alper. Y [4] calculated the heat exchanger by smooth tube, the result showed that pressure and heat loss was unchanged. Another study about the heat transfer on aluminum smooth tube was carried out by Hussein. A.M et al. [16]. In this study, the change of the rate of tube length (L) and tube diameter (D) was taken experimental to determine the convection effects and heat transfer coefficient. The evaluation of heat transfer and design the heat exchanger was based on the correlation between the Nusselt, Rayleigh, Reynolds, and Richardson number. Meyer. J.P et al. [20] studied about the enhancement of convection heat transfer for smooth tube though the change of flow properties, the study also showed that the pressure loss in case of using smooth tube is small due to low friction. Sherrow. L et al. [23] presented the effects of tube shape or adding the deep to the tubes exterior surfaces. The results showed that, the tube shape and arangement affected drammatically the augmentations of heat transfer in comparison with smooth surfaces. Moreover, Bouris. D et al. [9] also studied about the using of non-circular tubes with elliptic shape. The higher heat transfer, lower pressure loss was presented by this study. Daloglu. A et al. [10] presented the experimental results about the influence of cross-sectional shape with circle or square on the heat exchanging, either flow speed or tube shape resulting in high Reynolds numbers also increased the heat transfer coefficient, however in all tube shape, circle-smooth tube was used the most because easy design and fabrication.

The results from above researches showed that, the solutions of TEG, ORC, ETC, compressor-turbocharger, Stirling engines, exhaust boiler, absorbent refrigerator for recovering the WH from diesel engines are popular. However, the WH utilization from small diesel engines for heating up bio-oils in order to improve their properties equally to the properties of diesel fuel was almost not mentioned, although bio-oils are renewable and be able to rival to fossil fuels [14]. Thus, this paper presents the methods of utilizing the exhaust gas energy from small diesel engines to heat up bio-oils, using heated bio-oils as the alternative fuel not only increases the heat efficiency of engines but also reduces the pollution emission. Simultaneously, this paper gives a method of calculating and designing the exhaust gas heat exchanger by smooth tube to heat up bio-oils but ensure the normal working ability of the engines without occurring the resistance in the exhaust pipe.

1.1. Exhaust gas energy from diesel engines:

WH is heat generated by the fuel combustion or chemical reaction. In the time of engine run, four sources of WH such as exhaust gas, cooling oil/water/liquid, lube oil, and turbocharger are dissipated to the atmosphere from the engine. WH depends on not only the temperature of the waste heat gases, but also mass flow rate of exhaust gas of engines. Exhaust gas temperature of diesel engines after leaving the engine are as high as 450 -600[degrees]C. Consequently, the higher the exhaust gas temperature is, the higher the heat value is, however, the temperature of exhaust gases are limited by the laws of thermodynamics. Total energy from diesel engines is shown in the Figure 1.

For diesel engines in the ships that are bigger than 3000 hp of the power, the utilizing of exhaust gas energy is much more interested due to the space of engine room is large and exhaust gas heat energy is high. The exhaust gas energy of these engines can be recovered through the system of auxiliary--exhaust gas boiler or turbo--compressor aimed to heat up heavy fuel or turbochargers. However, small power engines, especially the diesel engines with less than 100 hp of power, it is not occasional to arrange the devices for utilizing the exhaust gas energy. In automobile diesel engines, significant amount of heat energy of exhaust gas (about 35% of the thermal energy) is released into the environment. The amount of such losses, they are recoverable at least partly or greatly and depend on the engine load and engine speed. Among various advanced methods, exhaust energy recovery for automobile diesel engines are proved to improve fuel consumption, reduce CO2 and other harmful emissions.

1.2. Benefits of WH recovery from diesel engines:

WH recovery from diesel engines brings many big benefits not only high power engines, but also smaller engine. Benefits from WH recovery may be divided into direct or indirect benefits.

Direct benefits: Recovery of WH from diesel engine may affect on the efficiency of combustion process due to it increases the obtained total energy, hence WH recovery is considered a solution with lower costs, cutting emissions, and especially, increasing the EEDI (Energy Efficiency Design Index) for the ship.

Indirect benefits: Some indirect benefits from WH recovery are included the pollution reduction, the device size reduction because of reducing the total fuel consumption, reduction in energy consumption for auxiliary devices such as boiler, compressor.

2. Properties of bio-oils:

Chemically speaking, bio-oils included vegetable oils and animal fats after removing the water, ash, and free acid are esters of fatty acid. In this study, straight coconut oil (SCO) is heated by exhaust gas energy through the heat exchanger and is used as fuel. The ASTM D1298 standard procedures were used to measure density, the ASTM D 445 standard was used to measure kinematic viscosity and Du Nouy ring method with a tension meter based on the ASTM D971 standard was used to measure surface tension of the SCO that is one of bio-oils. The physicochemical properties of SCO are presented in the Table 1.

It is observed that, HHV of SCO is 5-8% smaller than that of diesel fuel. However, the KV of the SCO is 7-10 times higher than that of diesel fuel. Therefore, heating SCO up to the suitable temperature in order to the KV of SCO be close to diesel fuel' one is necessary. The experimental test result about the relationship between the KV of SCO and temperature shows that, SCO heated up 100[degrees]C-110[degrees]C (HSCO) will satisfy the requirements of diesel fuel based on Vietnamese standard 5689-2005 and 1:2009.

3. Design the bio-oils heating system:

The bio-oils heating system by exhaust gas energy is designed based on the principles of ensuring that the system has all the basic functions such as fuel storage, supplying, filter, safety. The theoretical basis for calculating the fuel heating system is based on the equations of energy balance and heat transfer to guide the design and manufacture the bio-oils heating system. The calculation method for device utilizing exhaust gas energy (DUEGE) is shown in Figure 2.

3.1. Exhaust gas energy:

The diesel engine D243 is used to calculate and design the heating system by utilizing the exhaust gas energy in order to heat up SCO from room temperature to 100oC (HSCO). The technical parameters of engine D243 can be briefly featured as Table 2.

Exhaust heat loss of diesel engine D243

Compression ratio ([epsilon])

[epsilon] = [V.sub.[epsilon]] + [V.sub.s] / [V.sub.c] hence [V.sub.c] = [V.sub.s] / [epsilon]-1 (2)

Where [V.sub.s] = [[pi][D.sup.2]/4] S (3)

Therefore [V.sub.c] = 7.56.[10.sup.-5] [m.sup.3]

Total volume [V.sub.t] = [V.sub.c] + [V.sub.s] = 12.63 x [10.sup.-4] [m.sup.3] (4)

Mass flow rate of SCO fuel: [m.sub.f] = [g.sub.e] * [N.sub.e] = 4.67 g/s (5)

Volume rate v = [V.sub.s] x n = 21.75 x [10.sup.-3] [m.sup.3]/s (6)

Volume efficiency

[[eta].sub.v] = [m.sub.a]/[[rho].sub.a] . [eta] .. [V.sub.s] (7)

Where: Efficient volume [[eta].sub.v] is 0.8 to 0.9;

Density of HSCO fuel [[rho].sub.f] is 0.8639 g/[cm.sup.3]; lower heating value of HSCO is 35.85 MJ/kg;

Density air fuel [rho]a is 1.167 kg/[m.sup.3];

Specific heat of exhaust gas [C.sub.p] is from 1.1 to 1.25 KJ/kg.K

Therefore [m.sub.a] = [[eta].sub.v]*[[rho].sub.a]*N*[V.sub.s] = 1.24 kg/min = 20.67 g/s and mass flow rate of exhaust gas [m.sub.e] = [m.sub.f] + [m.sub.a] = 25.34 g/s

The WH energy contained in the exhaust gas depends on both the temperature and the mass flow rate of the exhaust gas. The funtion of heat loss for exhaust gas in diesel engines is given:

[Q.sub.ex] = [C.sub.[rho]] x [m.sub.e] x [DELTA]T (8)

Where, [Q.sub.ex] is the exhaust gas energy (kJ/s);

[m.sub.e] is the mass flow rate of exhaust gas (kg/s);

[C.sub.p] is the isobaric mass heat capacity of exhaust gas (kJ/kg.K);

[DELTA]T is the Kenvin temperature gradient (K).

Moreover, percentage of recovered WH from exhaust gas may be calculated:

[eta] = ([Q.sub.u]/[Q.sub.ex]) x 100% (9)

Exhaust gas temperature of diesel engine D243 in ([T.sub.ex-in]) and out ([T.sub.ex-out]) DUEGE, isobaric mass heat capacity of exhaust gas are plotted in Figure 3, and Figure 4, respectively.

It is found that, bio-oils are the fuel rapidly growing in use, and it should have good fluidity, low viscosity and good atomization which can only possible by preheating. There are many researchers used heating method to heat up bio-oils aiming at direct using in diesel engines, Acharya S.K et al. [1] used kusum oil for small diesel engine and shown that viscosity was close to diesel's by preheating to 100-130[degrees]C. P. P. Sonune et al. [25] used mahua oil and concluded that at the temperature above 100oC the viscosity reached to ASTM limits. Besides, R Ram Rattan et al. used mustard oil that was preheated up to 130[degrees]C to get the same density as that of diesel. R. Raghu et al. [26] used rice bran oil (RBME) and denoted that its viscosity was closer to diesel's as heated to 158[degrees]C. Soma C et al. [24] used waste vegetable oil which was preheated to 100[degrees]C and its viscosity became close to that of diesel. Oza N.P et al. [21] also denoted that jatropha and neem oil is preheated to overcome higher viscosity and lower volatility associated with biodiesel. A.T. Hoang et al. [14] compared the engine performance and thermal efficiency as using biodiesel, diesel oil and preheated vegetable oil.

SCO-direct heating system by utilizing the exhaust gas energy can be applied in small ship. The advantages of this method include high thermal performance and simple. However, the reliability is low, this heating system only works when the main engine run, causing the hydraulic impedance in the exhaust pipe and installation difficulties. Further, it is necessary to ensure the exhaust gas temperature out of the heat exchanger higher than 200[degrees]C in order to avoid the dew point corrosive phenomenon. The exhaust gas energy of diesel engine D243 depending on engine speed is shown in Figure 5.

RESULTS AND DISCUSSION

The device of utilizing exhaust gas energy is calculated and designed in case of the diesel engine working at 90% of engine load and 1500 rpm of engine speed. The redundancy heat energy of exhaust gas will be adjusted to expel into the environment by the by/pass valve. The 3D principle diagram of SCO heating system by utilizing, recovering the exhaust gas energy and integrating electricity energy used as engine starts is shown in Figure 6.

The calculation results about the heat exchanger for heating SCO based on the heat balance equation [Q.sub.ex] = [Q.sub.absorb] are given in the Table 3, and the model of smooth tube--heat exchanger (SPHE) utilizing exhaust gas energy is shown in Figure 7. Picture 1 shows the results of fabrication and installation for SCO heating system by integrating exhaust-electricity energy with SPHE on diesel engine D243 in the laboratory.

After calculating and designing the heat exchanger and due to the heat exchanger is placed in the exhaust pipe of diesel engine, therefore the determining of exhaust back pressure is necessary to ensure that the utilizing of the exhaust gas energy by heater to heat up SCO does not effect on the normal exhaust process of diesel engine. The parameters of the resistance in the exhaust pipe while the engine working at 75% and 100% of load are given in the Table 4.

Table 4 shows that, total resistance of heater to exhaust gas current satisfies the standard resistance at 75% and 100% of engine load, therefore, the heater can be used in order to heat up SCO. The SCO heater after calculating and designing is installed on the test-bed. Then, the HSCO is used as fuel in the diesel engines D243.

After designing and calculating, the DEUGE is simulated by ANSYS FLUENT. The simulating results are shown in Figure 8, Figure 9 and Figure 10.

From Figure 8, Figure 9 and Figure 10, it can be seen that, at 10% of load, the exhaust gas temperature and exhaust gas energy are low, the ability of utilization is still low. SCO fuel temperature out of heater almost constant and exhaust gas temperature is higher than 221[degrees]C and therefore, it is also higher than dew point temperature hence does not cause corrosion of equipment. At 75% and 100% of load with 1500 rpm of engine speed, temperature and higher temperature exhaust gases should be able to take advantage of higher temperatures. Exhaust gas temperature out of from heater is 376[degrees]C and 474[degrees]C, respectively. Besides, at 75% and 100% of load with 2000 rpm of engine speed, exhaust gas temperature out of from heater is 390[degrees]C and 510[degrees]C, respectively. At the low load, the heat energy utilized is lower, so SCO temperature is nearly unchanged. However, at 75% and 100% of load, the temperature of HSCO is 103[degrees]C and 105[degrees]C respectively.

The results of calculation and simulation show that the calculation and design of the DUEGE for heating up bio-oils does not affect the normal working in intake-exhaust stroke of the engine even as the engine is running at small speed and low load. In addition, the exhaust gas temperature of the engine out of DUEGE is always higher than the dew-point temperature, so the corrosion of the heat exchanger mounted on the exhaust pipe by acid will not occur. The average error between the experimental and simulation results is 9%, the exhaust gas energy can be optimized when the engine is operated at speed higher than 1500 rpm and over 50% of the load. At low loads, the exhaust gas energy is low due to the low exhaust gas temperature and mass flow, the SCO fuel temperature out of the heat exchanger is almost unchanged. Therefore, to heat SCO and maintain SCO's temperature at the right temperature, electrical energy or traditional diesel fuel should be used in this case. Thus, the bio-oil heating system by exhaust gas heat can only be fitted with diesel engines using dual fuel systems or integrated electric power.

CONCLUSION

Utilization of exhaust gas energy of diesel engine to improve the disadvantages of bio-oils and use heated bio-oils as fuel is necessary to improve the heat efficiency of diesel engine and reduce the environmental pollution. However, up to now, almost exhaust gas energy of diesel engine in the ocean ship with large engine is utilized primarily by heat exchanger such as exhaust boiler, compressor-turbocharger. Meanwhile, in Vietnam, small diesel engines (power <100 HP) on the inland fleet of ship or vessel are very large and the bio-oils source is abundant. Thus, the complex between exhaust gas energy recovery and using bio-oils as fuel for small diesel engines will bring big benefits, it will solve the matters related to energy saving, diversification of fuel source, pollution reduction, economic improvement. The results of this paper will orientate to calculate, design, fabricate and install the heat exchanger to recover the exhaust gas heat of diesel engines in small ship/vessel, generator or agricultural machinery, and convert the small diesel engines into operating with bio-oils. In next research, the effect of temperature on the heat exchanger strength will carry out and the power, emission characteristic of diesel engine fueled with bio-oils will be also mentioned aiming at proving the applicability the bio-oil heating system by exhaust gas energy.

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[30.] Zhang, C., G.Q. Shu, H. Tian, H. Wei, G. Yu, Y. Liang, 2014. Theoretical Analysis of a Combined Thermoelectric Generator (TEG) and Dual-loop Organic Rankine Cycle (DORC) System Using for Engines' Exhaust Waste Heat Recovery. SAE Technical, 2014-01-0670.

Anh Tuan Hoang

Vice Dean, Faculty of Mechanical Engineering, Ho Chi Minh University of Transport, Ho Chi Minh city, Vietnam

Received 12 May 2017; Accepted 5 July 2017; Available online 28 July 2017

Address For Correspondence:

Anh Tuan Hoang, Faculty of Mechanical Engineering, Ho Chi Minh University of Transport, No.2, D3 Road, Ward 25, Binh Thanh District, Ho Chi Minh city, Vietnam.

Tel: +84-904-317-584; E-mail: anhtuanhoang1980@gmail.com_

Caption: Fig. 2: Calculation method for DUEGE

Caption: Fig. 3a: Relationship between engine load and exhaust gas temperature at 1500 rpm

Caption: Fig. 3b: Relationship between engine load and exhaust gas temperature at 2000 rpm

Caption: Fig. 4: Relationship between exhaust gas temperature and isobaric mass heat capacity (CP) at 100% load

Caption: Fig. 5: Relationship between engine load and exhaust gas energy at 1500 rpm and 2000 rpm

Caption: Fig. 6: The 3D figure of SCO heating system by integrating exhaust-electricity energy

Caption: Fig. 7: Model of SPHE

Caption: Fig. 8a: Input and output exhaust gas temperature and SCO at 10% of load and 1500 rpm

Caption: Fig. 8b: Input and output exhaust gas temperature and SCO at 10% of load and 2000 rpm

Caption: Fig. 9a: Input and output exhaust gas temperature and SCO at 75% of load and 1500 rpm

Caption: Fig. 9b: Input and output exhaust gas temperature and SCO at 75% of load and 2000 rpm

Caption: Fig. 10a: Input and output exhaust gas temperature and SCO at 100% of load and 1500 rpm

Caption: Fig. 10b: Input and output exhaust gas temperature and SCO at 100% of load and 2000 rpm

Caption: Picture 1: SCO heating system by integrating exhaust-electricity energy with SPHE
Table 1: Physicochemical properties of SCO at room temperature

Properties                 Methods       Unit           Result

Lower heating value, LHV   ASTM D 240    kJ/kg          35.85
Cetan number, CN           ASTM D 976    --             40
Density, D                 ASTM D 1298   g/[cm.sup.3]   0.9103
Kinematic viscosity, KV    ASTM D 445    cSt            35.3
Surface tension, ST        ASTM D 971    N/m            0.0322

Table 2: Technical parameters of engine D243

Description                                    Unit     Parameter

Power, [N.sub.e]                               HP       80
Revolution, n                                  rpm      2000
Bore, D                                        mm       110
Stroke, S                                      mm       125
Compression ratio, [epsilon]                   --       16.7:1
Specific fuel consumption of SCO, [g.sub.e]    g/HP.h   210

Table 3: Design parameters of the heat exchanger

Parameters                      Sign

Diameter of SCO tubes           [d.sub.2]/[d.sub.1]
Heat coefficient of SCO         [[alpha].sub.2]
Heat coefficient of             [[alpha].sub.1]
  exhaust gas
Thermal conductivity            [lambda]
  coefficient of tubes
  material
Heat transfer coefficient       k
Heat transfer surface square    F

Parameters                      Unit            Result

Diameter of SCO tubes           mm/mm           12/8
Heat coefficient of SCO         W/[m.sup.2].K   25.3
Heat coefficient of             W/[m.sup.2].K   78.2
  exhaust gas
Thermal conductivity            W/m.K           35.5
  coefficient of tubes
  material
Heat transfer coefficient       W/[m.sup.2].K   19.08
Heat transfer surface square    [m.sup.2]       0.04

Table 4: Parameter of the resistance in the exhaust pipe

Parameters                       Unit           75% Ne        100% Ne

Reynolds of exhaust gas,                        1982          2126
 [Re.sub.eg]
Density of exhaust               kg/[m.sup.3]   0.491         0.415
  gas, [D.sub.eg]
Friction coefficient                            0.015         0.0297
  of parallel pipes
  cluster, [xi]
Friction resistance of heater    Pa             152           421
  to exhaust gas current,
  [DELTA][p.sub.fr]
Local drag coefficient of                       2.5           3.8
  parallel pipes cluster,
  [[xi].sub.ld]
Local resistance of heater to    Pa             39.7          47
  exhaust gas current,
  [DELTA][p.sub.lr]
Total resistance of heater to    Pa             191           468
  exhaust gas current,
  [DELTA][p.sub.tr]
Standard resistance,             Pa             1472 / 3924
  [DELTA][p.sub.sr]

Fig. 1: Total energy in diesel engines

Exhaust gas energy               30%
Brake power                      35%
Cooling water/oil and lube oil   30%
Radiation                        5%

Note: Table mad from pie chart.
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Author:Hoang, Anh Tuan
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Date:Jul 1, 2017
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