外文原文-橘子皮干燥动力的研究和强制对流太阳能干燥器的发展现状.pdf

Study of orange peels dryings kinetics and development of a solar dryer by forced convection Romdhane Ben Slama a Michel Combarnousb aHigher Institute of Applied Sciences and Technology of Gabes Road of Medenine 6029 Gabes Tunisia bUniversity of Bordeaux Laboratoire TRansferts Ecoulements FLuides Energe tique TREFLE ENSAM 33400 Talence 33405 France Received 22 April 2010 received in revised form 10 November 2010 accepted 2 January 2011 Available online 1 February 2011 Communicated by Associate Editor I Farkas Abstract The solar drier project aims at using solar energy as heat source frequent in the area for the drying of perishable products Solar drier does not degrade any more the dried products with the manner of the products dried at the natural sun The drying unit is composed mainly of a solar air collector and a drying chamber The transformation of the solar radiation into heat is done thanks to the solar collector whose eff ectiveness is increased by the addition of suitable baffl es in the mobile air vein The effi ciency of the collector reaches then 80 The hot air on the outlet side of the collector arrives in the drying chamber where the heat transfer with the product to be dried is done by convection The drying kinetics study shows that in addition to the dependence of the temperature and air velocity of drying the rate of drying also depends on sectioning on the product to dry and mainly of the product surface in contact with the drying air Thus the moisture content in wet basis is reduced from 76 to 13 in one day Then we obtains dried products in a healthy way and the so frequent sand wind in the area does not degrade any more the dried products quality at the manner of the products dried with the free air The total effi ciency of the drier reached 28 Published by Elsevier Ltd Keywords Solar energy Drying Orange Convection Kinetics Effi ciency 1 Introduction Solar energy is very abundant in Tunisia particularly in the south So it would be judicious and profi table to use it more and more in the fi eld of drying energy safety and environment protection Two types of driers one direct and the other indirect drier whichispresentedherewereconceived carriedoutandtested at the National School of Engineers of Gabe s Tunisia in ordertodryagro alimentaryproductssuchasoforangepeels The orange peels are interesting for their wealth of vita min C The studies showed that they lower the cholesterol level in the blood In our area the south of Tunisia it is common to dry dates spices peppers fi sh etc Previously a study of the drying kinetics has been carried out Tests were carried out in the full heat of the sun at the same time for the indirect and direct drier The solar types of driers are very varied in the world We can mention among them Industrial drier tunnel type with chimney natural con vection the wide surface collecting solar energy makes it possible to collect a thermal energy from where the industrial use of this type of driers Turhan 2006 Janjaietal 2008 Ferreiraetal 2008 Animprovement of them is giving by Sethi et al Sethi and Arora 2009 Family driers Direct drier with forced ventilation Gauhar etal 1998 Hossain and Bala 2007 Gbaha et al 2007 Indirect drier with solar chimney Hachemi et al 1998 Pangavhane et al 2002 Lahsani et al 2004 Jain 2005 0038 092X see front matter Published by Elsevier Ltd doi 10 1016 j solener 2011 01 001 Corresponding author E mail address BenSlama Romdhane yahoo fr R B Slama Available online at Solar Energy 85 2011 570 578 Indirect drier with forced ventilation Ben Slama etal 1996 BenSlamaandBouadallah 1996a b Hawlader 2004 Fadhel et al 2005 Jamali et al 2004 Machlouch et al 2006 Zhimin et al 2006 Mixed mode natural convection solar crop dryer Forson et al 2007 The paper of Jairaj Jaira et al 2009 is a review of many popular varieties of solar dryers The types of drained products are also varied Wood it is put in a room and receives the hot air of a tunnel greenhouse Bentaieb et al 2008 Thanks to the diff erence in pressure the air circulation is ensured by a chimney Lemon tomato chilli peppers apricot grape meat fi sh herbs and spices Chen et al 2005 Togrul and Pehlivan 2002 Kamil et al 2006 Janjaia et al 2008 Orange peel object of this study which concerns the drying kinetics study and the design of the tested drier 2 Drying kinetics study 2 1 Object The drying kinetics curves are the key factor of any prac tical modelling for a dryer So they are present in various studies Ait Mohamed et al 2008 Hadri et al 2008 for products such as pepper sardine banana wood carrot etc We are interested here in the orange peels and traced their curves of drying kinetics These curves have been obtained with a drying labora tory cell with a controlled procedure for temperature pres sure air fl ow moisture and speed of drying 2 2 Results of the drying kinetics The product dries more quickly at the beginning than at the end which is due to the presence of surface water in the fi rst case This speed is the highest since the product is frag mented squares of 5 5 mm thus comprising the highest heat transferringsurface Inthesameway thisrateofdrying is proportional to the temperature of heating Fig 1 and to the air velocity of drying Fig 2 The last curve constitutes the curve characteristic of the orange peel drying In Fig 3 on the basis of an initial moisture from 3 to 4 kg water kg DM according to time the moisture content and the rate of drying decrease to be cancelled at the end of 12 000 s approximately 3 h 300 for speeds varying from 1 24 to 2 1 m s at an air drying temperature of 75 C For the curves of drying speed presented in Fig 4 we observe that the curves cross because the more the air velocity is raised the faster drying is Therefore at the end of this time 3 h 30 the product is already dry con trarily to the cases with lower air velocities which require more time of drying see Fig 5 3 Design of the tested drier The indirect drier design includes three distinct parts A solar air collector equipped with baffl es Ben Slama 1987 2007 BenSlamaetal 1996 dimensions2 m 1 m F drying enclosure dimensions 70 cm 70 cm 70 cm Nomenclature AHtime angle d C specifi c heat Jkg 1K 1 DH diff use solar radiation incident on the horizontal plane W m 2 D i diff use solar radiation incident on the collector W m 2 GHtotal solar radiation incident on the horizontal plane W m 2 G i total solar radiation incident on the collector W m 2 hsolar latitude d isolar collector slope d ID direct solar fl ux Wm 2 Lvlatent heat of vaporization J kg 1 mmass evaporated water kg Qreceived solar energy kW h Qvvolume throughput m3s 1 Ssurface collecting m2 Tiinitial temperature of the product K Taambient temperature K W Xmoisture content of the product kg kg DM uangle formed by the normal with the collector and the incidental solar rays d Vair velocity m s d t declination of the sun d latitude of place d aalbe do g effi ciency DT0calculated duration of the day h Indices i ininitial fend uuseful rereceived eentry sexit R B Slama M Combarnous Solar Energy 85 2011 570 578571 An electric fan to ensure the forced circulation of the hot air coming from the fl at plate solar collector of 50 W they can be provided by photovoltaic devices 3 1 The solar air collector The collector is equipped with baffl es which favour the turbulence and thus the heat transfer The effi ciency is increased as well see Fig 6 Three confi gurations were used Collector with the best baffl es appeared in our refer ences i e of the transverse and longitudinal mixed baffl es Mixed baffl e collector however the transverse baffl es are slightly tilted in the direction of the fl ow to reduce the pressure losses Collectorwithout baffl estobeusedfor comparison y 0 013x2 0 955x 0 028 R 0 999 y 0 093x2 0 383x 0 168 R 0 993 y 0 144x2 0 004x 0 128 R 0 995 0 0 5 1 1 5 2 2 5 3 3 5 4 4 5 01234 dX dt kg kg h X kg eau kg MS Temperature 75 CTemperature 66 C Temperature 57 CPoly Temperature 75 C Poly Temperature 66 C Poly Temperature 57 C Fig 1 Rate of drying function of the moisture content and the air temperature Air velocity 2 1 m s y 0 010 x2 0 963x 0 024 R 0 999 y 0 010 x2 0 719x 0 049 R 0 999 y 0 033x2 0 535x 0 091 R 0 998 0 0 5 1 1 5 2 2 5 3 3 5 4 4 5 00 511 522 533 54 dX dt kg kg h X kg water kg DM Air velocity 2 1 m sAir velocity 1 5 m s Air velocity 1 24 m sPoly Air velocity 2 1 m s Poly Air velocity 1 5 m s Poly Air velocity 1 24 m s Fig 2 Rate of drying function of the moisture content and air velocity Temperature of the air 75 C 572R B Slama M Combarnous Solar Energy 85 2011 570 578 The collector has two air ducts One is between the glazing and the absorber with the stagnant air there The other is between the absorber and the insulator comprisingsome baffl esandinwhichtheair circulates The baffl es are placed on the insulator and or are fi xed at the absorber see Fig 7 3 2 Drying enclosure At the exit of the collector the air crosses the drying chamber and transfers heat to the product to be dried To allow the air fl ow the two shapes of drying chamber are proposed allowing one run out in meander without dead zones see Fig 8 The drying chamber comprises sev eral trays Each one can contain up to three kg of product to dry Products are disposed on the trays inside the drier in such a way that the air fl ow presents some meanders in order to obtain a relatively high heat transfer In addition the fl ow in the drying enclosure is favored when the fl ow passes from simple meander to double mean der Thus the air is distributed allowing a more uniform drying what avoids having still wet zones and others which are quite burned The effi ciency increases from 20 to 25 especially when the loading is maximum higher than 10 kg For the low masses of product to be dried the air is not constrained y 3 941e 2E 0 x R 0 988 y 3 741e 3E 0 x R 0 988 y 2 84e 2E 0 x R 0 980 0 0 5 1 1 5 2 2 5 3 3 5 4 4 5 0200040006000800010000120001400016000 X kg water kg MS Time s V 1 24 m sV 2 1 m s V 1 5 m sExpon V 1 24 m s Expon V 2 1 m s Puissance V 1 5 m s Fig 3 Water content function of the time and air velocity Temperature 75 C y 3 880e 3E 0 x R 0 993 y 2 487e 2E 0 x R 0 990 y 2 136e 2E 0 x R 0 990 0 0 5 1 1 5 2 2 5 3 3 5 4 4 5 0300060009000120001500018000 dX dt kg kg h Time s Air velocity 2 1 m sAir velocity 1 24 m s Air velocity 1 5 m sExpon Air velocity 2 1 m s Expon Air velocity 1 24 m s Expon Air velocity 1 5 m s Fig 4 Drying speed function the time and air velocity Temperature 75 C Fig 5 Photograph of the indirect drier R B Slama M Combarnous Solar Energy 85 2011 570 578573 to follow the overall trajectory but a straight fl ow in the open space left by the product 3 3 Operation The ambient air which enters through the bottom of the collector is heated there by solar energy Owing to the baf fl es the heat transfer is increased and the air at the collec tor exit is at a suffi ciently high temperature It passes then in the drying chamber where it transfers heat to the prod uct to be dried and which is loaded with moisture Then it is evacuated outside by forced ventilation ensured by a low power electric fan 4 Tests results Measurements are related mainly to the effi ciency of the collectors alone on the one hand and the entire drier on the other hand We measure the mass of the product to be dried at the beginning and at the end of the test 4 1 Measurement of the solar radiation The energy received per day is given owing to measure ments of diff use DH and total GH horizontal radiation from the weather station located at the neighbouring mili tary base The conversion of measurements on the horizon tal level towards the tilted plan of the collector is carried out as follows The total radiation incident on the tilted collector is the sum of the direct and diff use radiations then G i Gh Dh cos i sin i tg h D i 1 which can be also written under the form G i Gh Dh sin i h sin h D i with 2 D i 1 cos i 2 Dh 1 cos i 2 Gh a 3 The solar latitude h is given at any moment by its sinus sin h sin u sin d cos u cos d cosAH 4 Fig 6 Detail of the solar air collector showing a its composition b the straight baffl es c oblique baffl es and d the fl ows generated a b Fig 7 Drying enclosure a Air fl ow in meander b Double air fl ow in meander 574R B Slama M Combarnous Solar Energy 85 2011 570 578 4 2 Measurement of drier energetic effi ciency Temperatures are measured with thermocouples iron constantan precision 0 5 C For the weight a balance of precision 0 01 g Drier energetic effi ciency is giving by ge Qu Qrewith 5 Qu m C Ti Ta Lv and 6 Qre Z t2 t1 G i dt 7 As it is known that the rate of drying is faster at the begin ning than at the end of drying then one fi xed constant fi nal moisture of 15 The hygrometric effi ciency is also evaluated gh ge Wi Wf Wi 8 and the corrected effi ciency with Wf 15 gc ge Wi Wf Wi 15 9 As it is known that the speed of drying is faster at the beginning than at the end of drying then one fi xed con stant fi nal moisture of 15 4 3 Tests curves Usually for the air solar collectors one can plot the effi ciency curves according to the air fl ow m3 h m2 Fig 8 and according to the benefi t normalized DT G i m2 C W Fig 9 Temperature is non constant but is depending with solar energy and air fl ow rate Even for the modest fl ows 35 m3 h m2 the effi ciency reaches 70 for the best baffl es and 46 for the collectors without baffl es This diff erence is due to the diff erence of the convective exchanges created by turbulence between the caloporting air and the absorber it is even due to the presence of baffl es 4 3 1 The collector effi ciency The energetic effi ciency is presented in Fig 10 and is given by the expression g QV q C DT G i S 10 According to the provision of the baffl es used straight or inclined the effi ciency varies In comparison with the col lector without baffl es the oblique baffl es in the direction of the fl ow have a better effi ciency of 25 Ben Slama 1987 2007 Ben Slama et al 1996 Fig 9 shows that for an effi ciency of 60 taken as ref erence the rise in temperature would be only 40 C for the case without baffl es 63 C with the straight baffl es and 80 C with oblique baffl es under a sunny heat fl ux of 1000 W m2 4 3 2 Effi ciency of the drier By analogy with the solar air collectors one gives two types of curves of effi ciencies for the solar drier according to the initial mass of the product to be dried Fig 10 and function of the benefi t normalized decrease of the moisture content Fig 11 Ben Slama et al 1996 Ben Slama and Bouabdallah 1996a Machlouch et al 2006 For a mass of 8 kg product to be dried the effi ciency increases from 20 to 30 approximately with the use of slightly oblique baffl es For an hygrometric effi ciency of 20 the decrease of moisture content would be only 3 in the case with straight baffl es and 13 for oblique baffl es This is due to the fact that the air reached higher temperatures in the presence of baffl es 4 3 3 Pressure drops For an empty drier the pressures drops increase with the air fl ow However they are higher in the case of straight baffl es than oblique ones Indeed these latter being tilted in the direction of the fl ow reduce the dead zones and facilitate the cooling air fl ow The loading of the drier by the product increase the pressure loss by more than 50 Lahsani et al 2004 see Fig 12 4 3 4 Interpretations The curves obtained by the drying kinetics study show that 0 10 20 30 40 50 60 70 80 90 051015202530354045 Efficiency Air flow rate m3 h m Oblique bafflesStraight bafflesWithout baffles Fig 8 Effi ciency of the collector function of the air fl ow Temperature is necessarily variable R B Slama M Combarnous Solar Energy 85 2011 570 578575 The moisture of the product decreases with time much more quickly at the beginning than at the end of drying because at the beginning the presence of water is rather on the surface The rate of drying is proportional to both drying air velocity and drying temperature The sectioning of the orange peels increases the dry ing speed y 0 055x2 6 531x 108 0 R 0 998 y 0 019x2 5 276x 316 1 R 0 977 y 0 082x2 10 56x 359 8 R 0 851 0 10 20 30 40 50 60 70 80 405060708090 Efficiency 1000 Ts Te G i m C W Obliques bafflesStraight baffles Without bafflesPoly Obliques baffles Fig 9 Solar collector effi ciency according to the standardized profi t y 0 098x2 3 877x 5 468 R 0 975 y 0 085x2 3 674x 1 801 R 0 957 y 0 175x2 0 749x 6 185 R 0 981 0 5 10 15 20 25 30 35 0123456789 Efficiency Initial mass kg Oblique bafflesStraight baffles Whithout bafflesPoly Oblique baffles Poly Straight baffles Poly Whithout baffles Fig 10 Effi ciency of the drier function of initial mass of the product to be dried 0 5 10 15 20 25 30 0510152025 W0 Wf Qreceived kg kgWM m kWh Hygrom tric efficiency Oblique bafflesStraight bafflesWithout baffles Fig 11 Effi ciency of the drier according to the standardized weight loss 576R B Slama M Combarnous Solar Energy 85 2011 570 578 The drying speed decreases with time and with the moisture of the product to be dried The pilot drier shows that the outlet temperature of the solar collector reached 92 C and that the temperature at the removal from the drying enclosure is still 50 C The drier using the best slightly oblique baffl e collector in the direction of the fl ow improves the effi ciency by 14 absolute compared to the drier using a collector with out baffl e With this latter one day is proved to be insuffi cient to lower the moisture content to a satisfactory threshold for the conservation of the product that is to say for example 15 However with the collector pro vided with oblique baffl es in the direction of the fl ow only one day of drying is suffi cient because the two conditions to allow a good drying are satisfi ed namely a temperature and a speed of air suffi ciently high The temperature of the air rises in the collector owing to the transverse and lon gitudinal baffl es which make it possible to multiply the air velocity and to support