Convective and evaporative heat transfer coefficients during drying of ivy gourd under natural and forced convection solar dryer

In the present work, a study on convective heat, mass transfer coefficients and evaporative heat transfer coefficient of the thin layer drying process of ivy gourd is performed. The experiment was conducted in three drying modes such as natural, forced convection solar dryer and open sun drying. The hourly data for the rate of moisture removal, sample temperature, relative humidity inside and outside the solar and ambient air temperature for complete drying have been recorded. The drying air temperature varied from 55, 65, 70 and 75 °C, and the air velocity was 1, 1.5 and 2 m/s. All the drying experiments had shown a falling rate period. The data obtained from experimentation have been used to evaluate the experimental constant values of C and n by simple regression analysis. Based on the values of “C” and “n”, convective and evaporative heat transfer coefficients for ivy gourd were determined. The average convective heat and mass transfer coefficients varied between 2.64 and 8.30 W/m2 °C and 0.0025 to 0.0076 m/s for temperature ranges, at the different air velocities, respectively. The average evaporative heat transfer coefficient for ivy gourd varied from 181.89 to 421.84 W/m2 °C. It was observed that convective and evaporative heat transfer coefficients increase with the increase in drying air temperature. The rate of increment of evaporative heat transfer coefficient is higher than the convective heat transfer coefficient. The intensity of heat and mass transfer during solar drying depends on the drying air temperature and velocity.


Introduction
Drying is a mechanism that involves heat and mass transfer from a source to a sink or the atmosphere. Solar drying is a method of drying that uses heat energy from the sun to dry food samples (Natarajan and Elavarasan 2019a). Ivy gourd is classified as a perennial herb or a widely found vegetable in parts of Southeast Asia and Africa. The samples typically last a week without preservation, whereas on drying, the shelf life can be increased to 2-3 weeks (Elangovan and Natarajan 2021a;Elavarasan and Sendhil Kumar 2021;Kumar et al. 2022). Thus, drying the food samples is the most suited cost-effective process to increase the shelf life while reducing the wastage of the samples. In this regard, Kumar et al. (2011) investigated the convective heat transfer coefficient of drying papad under sun drying and active mode dryer. It was observed that the convective heat transfer coefficients of drying papad varied from 1.54 to 1.56 W/m 2 °C under an active mode dryer. It was also observed that the convective heat transfer coefficients of drying papad varied from 2.11 to 5.52 W/m 2 °C under the sun drying method. Miketinac et al. (1992) developed five different simulating models for predicting the convective heat transfer coefficients during the drying of barley. It was observed that the convective heat transfer coefficients of dried barley for five different models varied from 43 to 59 W/m 2 K. It was also observed that the convective mass transfer coefficients of dried barley for five different models were found to be 1.08 × 10 −6 m/s. Kulkarni and Vijayanand (2012) investigate the effect of pretreatments of dried ivy gourd under a hot air dryer. It was observed that the dried ivy gourd was a good source of vitamin C. Anwar and Tiwari (2001a) experimentally determined the heat transfer coefficient of drying green peas, chilli, potato, chana, cauliflower and onion in open sun drying method at drying air temperatures varied from 25 to 50 °C. It was observed that the convective heat transfer coefficients of drying crops varied from 3.71 to 25.98 W/m 2 °C. It was also observed that the experimental error of open drying was found to be 35%. Zielinska and Markowski (2007) studied the convective heat and mass transfer coefficient of drying carrot slices under a fluidized bed dryer. It was observed that the heat and mass transfer coefficient of drying carrot slices varied from 13 to 38 W m −2 K −1 and 7 to 17 m s −1 . Prasad (2009) studied the convective heat transfer coefficients of drying three different agricultural samples, namely, gurjo, turmeric and ginger, under the open sun drying method. It was observed that the convective heat transfer coefficients of dried three different agricultural samples, namely, gurjo, turmeric and ginger, were found to be 3.9, 3.4 and 3.3 W/m 2 K. Kumar (2016) experimentally studied the convective and evaporative heat transfer coefficient of ginger under an active mode indirect solar cabinet dryer. It was observed that the convective and evaporative heat transfer coefficient of ginger was found to be 3.95 and 160.47 W/ m 2 °C. It was also determined that the collector efficiency of the active mode indirect solar cabinet dryer was found to be 14.5%. It was concluded that the experimental error was observed to be 20.87%. Goyal and Tiwari (1998) developed multiple regression equations for predicting the heat transfer coefficient for a thin layer of gram and wheat under a crop dryer. It was mentioned that the convective heat transfer coefficient for gram and wheat was found to be 9.62 and 12.68 W/ m 2 °C. It was also mentioned that the simple regression equation of the heat transfer coefficient for gram and wheat was found to be 9.67 and 10.85 W/m 2 °C. Zuritz et al. (1990) determined that the convective heat transfer coefficient of mushroom-shaped aluminium material in carboxymethylcellulose was between 548 and 1175 W/m 2 °C with average viscosity the ranges from 2.08 to 17.7 Pas. It was observed that the convective heat transfer coefficient increased with the flow rate and size of the particle and decreased apparent viscosity at 71 °C. Kumar (2013) evaluated the convective and evaporative heat transfer coefficient of papad under passive and active mode greenhouse dryer. It was found that the convective and evaporative heat transfer coefficient of papad under passive mode greenhouse dryer varied from 0.739 to 0.786 and 21.37 to 25.42 W/m 2 °C. It was determined that the mean values of convective and evaporative heat transfer coefficients of papad under active mode greenhouse dryer were found to be 0.759 W/m 2 °C and 23.48 W/m 2 °C. It was also reported that the experimental error varied from active and passive mode greenhouse dryer was in the range of 23.23-44.88%. Sahdev et al. (2017a) investigated the evaporative and convective heat transfer coefficient of groundnut under an active mode laboratory dryer. It was determined that the mean value of evaporative and convective heat transfer coefficient was found to be 35.08 and 2.48 W/m 2 °C. It was also determined that the experimental error varied from active mode laboratory dryer was found to be 42.55%. Prasad and Vijay (2005) presented the convective heat transfer coefficients of drying three products, namely, gurjo, turmeric and ginger, under the open sun drying method. It was observed that the convective heat transfer coefficients of dried three products, namely, gurjo, turmeric and ginger, varied from 1.57 to 3.85, 2.32 to 3.42 and 1.62 to 3.34 W/m 2 °C. Subramani et al. (2020) fabricated the natural and forced convection greenhouse solar dryer for drying two different samples, namely, ivy gourd and turkey berry. It was determined that the thermal efficiency of the greenhouse dryer was found to be 30.6%. It was concluded that the exergy efficiency of the dryer was found to be 0.09%. Tiwari et al. (2004) determined the convective heat transfer coefficients for the different mass of jaggery dried under the active and passive mode of the greenhouse solar dryer. It was observed that the convective heat transfer coefficients for the different mass of jaggery under passive and active mode dryer varied from 1.29 to 1.41 and 1.3 to 1.46 W/ m 2 Togrul (2003) experimentally determined the convective heat transfer coefficient of nine different food samples under the sun drying method. It was observed that the convective heat transfer coefficients of nine different food samples varied from 0.25 to 3.3 W/m 2 °C. Addis et al. (2009) investigate the effect of blanching treatment on ivy gourd and fenugreek under three different drying methods: open sun drying, hot air dryer and freeze dryer. It was observed that the blanching process does not affect the carotenoids level in the ivy gourd and fenugreek samples. Anwar and Tiwari (2001b) presented the convective heat transfer coefficients of six different food samples dried under an active mode convective dryer. It was found that the convective heat transfer coefficients of six different food samples varied from 1.3 to 12.8 W/m 2 °C. Sahdev et al. (2013) determined the convective heat transfer coefficients of drying corn kernels under an indoor active mode dryer. It was observed that the convective heat transfer coefficients of dried corn kernels varied from 1 to 1.04 W/m 2 °C. Kumar et al. (2012) studied the convective and evaporative heat transfer coefficients during heating of milk under natural convection. It was observed that the convective heat transfer coefficient of heating milk varied from 3 to 6.01 W/m 2 °C. It was observed that the evaporative heat transfer coefficient of heating milk varied from 16.09 to 95.16 W/m 2 °C. It was also determined that the experimental error was in the range of 17.88-38.21%. Kumar et al. (2015) evaluated the evaporative heat transfer coefficients of sensible heating of milk during khoa making. It was found that the evaporative heat transfer coefficient of sensible heating of milk increase by increase the operating temperature. It was observed that the evaporative heat transfer coefficient of sensible heating of milk varied from 11.43 to 86.81 W/m 2 °C. A summary of the above-cited literature review for different drying methods of fruit, vegetables and grains represented in Table 1.
Based on the above-cited literature review, it is identified the strong requirements for conducting an experiment on ivy gourd with appropriate heat transfer coefficients. The present study has been undertaken to determine convective heat and mass transfer coefficient and evaporative heat transfer coefficient of ivy gourd dried in three different forms. The values would be useful for predicting drying parameters and designing a suitable drier as in a variety of preparations; ivy gourd is used in dried form. The present work would be helpful in designing a better dryer for drying ivy gourd to its equilibrium moisture content.

Experimental setup
The experiment was performed in a single slope single basin solar dryer that works on natural and forced convection principles. The solar dryer has base dimensions of 1290 × 850 mm with varying heights of 500 mm and 260 mm on the larger and smaller side, respectively. The top of the dryer was covered with a pane glass of 4 mm thickness and an inclination of 10.2° (latitude of Karaikal) was given for the dryer. The position of the dryer was turned in the direction where appropriate sunlight can be received. For forced convection solar dryer setup blower (M4000B, Makita, India) is used for providing uniform air velocity. Relative humidity was measured by a digital hygrometer (Generic, HTC-1 SRISH, with the accuracy of ± 5% RH, India). The photographs of the experimental set-up for open sun drying, forced and natural convection solar dryer were shown in Fig. 1.

Sample preparation
The samples consist of 1 kg of ivy gourd, which is purchased from the local market in Karaikal. The samples were washed, dried and cut evenly with a thickness of 0.5 cm. The initial average diameter of the ivy gourd sample was 30.32 mm, respectively, as shown in Fig. 2.

Experimental procedure
The ivy gourd slices were placed evenly inside the dryer to obtain maximum efficiency. The data acquired during the entire course of the experiment was collected via a data acquisition system (Agilent 34972A, Keysight, India) (Kumar Natarajan et al. 2019). Eight K-type (with the accuracy of ± 0.1 °C) thermocouples were utilized to measure the temperature within the system. Five of them were used to measure the temperature of the absorber plate. Three of them were used to measure the drying air temperature at different locations. The ambient temperature was measured using a J-type thermocouple (with the accuracy of ± 0.1 °C) throughout the experiment. The global solar radiation was measured using a Hukseflux pyranometer (SR20-TI, Netherlands) with a 14.77 × 10 −6 V/(W/m 2 ) sensitivity. The ivy gourd slices were weighed periodically to record the reduction in moisture during the entire course of the experiment. The observation was given as follows.

Observation and thermal modelling
The moisture ratio of an ivy gourd is the ratio of the amount of moisture present in the gourd at any point of time to the amount of moisture content present initially in the ivy gourd (Li et al. 2020;Ahmad et al. 2021). It is a significant parameter that is monitored continuously throughout the drying process (Ullah et al. 2020).
The convective heat transfer coefficient for forced and natural convection can be defined as (Pitts and Sissom 1977;Nijmeh et al. 1998;Anwar and Tiwari 2001b;Tiwari 2002): For forced convection, the expression can be written as: For natural convection, the expression can be written as: The rate of heat utilized to evaporate moisture from the samples can be given as (Malik et al. 1982) On substituting (3) with (4) The evaporated moisture from the dryer was determined by dividing the above equation by latent heat of vaporization (λ) and multiplying by time interval ( t ) and area of mesh ( A m ).
(  K v X C(RePr) n P T i − P T c tA m = Z 1 and let 0.016P Taking logarithm on both sides of Eqs. (7a) and (7b) This was in the form of a linear equation, The values of m and C 0 were evaluated using the formulae of simple regression. The evaporative heat transfer coefficient was evaluated by using the following formula (4): The value of constants was obtained from Eq. 2. The physical properties like specific heat ( C v ), thermal conductivity ( K v ), density ( v ), viscosity ( v ) and partial vapour pressure (P(T)) were calculated using the following equations. where

Results and discussion
The experiment to dry ivy gourd samples was carried out over 3 days with changes in the velocity of the forced convection dryer (1, 1.5 and 2 m/s) along with natural convection and open sun drying to determine the moisture ratio, drying and absorber plate temperature, heat transfer coefficient, mass transfer coefficient and evaporative heat transfer coefficient of ivy gourd samples. The values of heat transfer, mass transfer and evaporative heat transfer coefficients are integral to the design of the dryer as it greatly influences the drying of the samples (Anwar and Tiwari 2001a). The results were obtained as follows.
The above figures observe the variations of the moisture ratio of forced, natural convection and open sun drying process for ivy gourd samples during the 3 days of the experiment. The equilibrium moisture for the samples was observed to be 10% which signifies the completion of the experiment. Figure 3 shows that on day 1, the samples placed under forced convection drying had the fastest drying time at 5 h due to the effect of uniform air circulation inside the dryer seconded by the natural convection drying at 7 h due to the effect of drying in an enclosed chamber followed by the open sun drying process at 9 h due to the lack of enclosed chamber or uniform air circulation during the course of drying. The moisture content of the agricultural samples decreased with respect to the drying period reported by Madan et al. (2014). A similar moisture content trend is observed in the work for solar drying of ivy gourd (Elangovan and Natarajan 2021b, a; Elavarasan and Sendhil Kumar 2022) and red banana (Elangovan and Natarajan 2021c; Elavarasan and Sendhil Kumar 2021), potato (Natarajan and Elavarasan 2019b), Poovan banana (Bhanu et al. 2021;Elavarasan et al. 2022a) and tomato (Elavarasan et al. , 2022b for same 5 mm thickness of samples and the temperature varied from 45 to 70 °C. It was clearly noted that the forced convection drying process was the best drying process for ivy gourd samples due to the uniform air velocity inside the dryer. Figures 4 and 5 show similar observations for the drying of ivy gourd samples on days 2 and 3, with variations observed only in the progression towards the completion of the experiment. However, there were no significant deviations in the drying time of the samples. Figure 6 observes the variation in the global solar radiation, the temperature of the atmosphere, drying air and absorber plate with the drying time of the ivy gourd samples under natural convection on day 1. The maximum   Figure 8 shows that the maximum and average drying air and absorber plate temperature were recorded as 67.83 °C, 55.24 °C and 73.42 °C, 59.38 °C and the maximum and average ambient temperature were recorded as 37.65 °C and 33.98 °C. The average and maximum global solar radiation were observed as 961.41 W/ m 2 and 778.61 W/m 2 . It was noted that the temperature of the absorber plate and drying air were proportional from the start to the end of the trials on every occasion and the variations of the drying temperatures and global solar radiation with the drying time of the samples for natural convection were similar on all three trials which were predominantly due to the unchanged drying conditions of the samples on all 3 days of the experiment. Figure 9 observes the variation of global solar radiation and temperature of drying air, absorber plate and atmospheric temperature of ivy gourd samples under forced convection on day 1. The maximum and average global solar radiation were recorded as 920.79 W/m 2 and 735.27 W/ m 2 . The maximum and average absorber plate and drying air temperature were recorded as 73.63 °C, 69.80 °C and 67.77 °C, 62.23 °C. The peak temperature of both drying air and absorber plate was observed during the 4th hour of the experiment. The average and maximum atmospheric temperature were recorded as 37.91 °C and 35.56 °C on the 1st day of the experiment. The velocity of the trial was fixed at 1 m/s and its effect on the temperature of drying air and absorber plate was that it showed a disproportional curve towards the end of the experiment. Figure 10 shows that the average and maximum atmospheric temperature and global solar radiation were recorded as 35.41 °C and 38.71 °C, 813.36 W/ m 2 and 968.18 W/m 2 . The maximum and average absorber plate and drying air temperature were recorded as 81.21 °C and 70.47 °C, 74.80 °C and 64.18 °C during the 2nd day of drying ivy gourd samples, where the velocity of the dryer was fixed at 1.5 m/s. This caused the temperature of the absorber plate and drying air to be proportional to each other until the end of the experiment. Figure 11 shows that the average and maximum drying air and absorber plate temperature were recorded as 59. 40 °C, 73.55 °C and 66.15 °C, 79.76 °C for the duration of the trial. The maximum and average atmospheric temperature and global solar radiation were recorded as 37.65 °C, 33.98 °C and 961.41 W/ m 2 , 778.61 W/m 2 during the 3rd day of the experiment. The velocity of the dryer was fixed at 2 m/s, which caused the curves of drying air and absorber plate to be proportional to each other after the 5th hour of the drying process. It was also observed that the increase in velocity increased the average drying temperature of the dryer though there was not much change in the temperature of the absorber plate for the drying process. A similar trend is observed by Gatea (2011) and Elangovan and Natarajan (2021b), which has shown a maximum drying air temperature of 72 °C and average solar radiation of 750 W/m 2 .
The rate of moisture removal plays a vital role in the computation of the convective heat transfer coefficient. The major parameters that affect them are the moisture content of the samples, their porosity and the size and shape of the samples (Akpinar 2004(Akpinar , 2005. Figure 12 shows the values of the heat transfer coefficient of ivy gourd under natural convection for 3 days of drying. The maximum value of heat transfer obtained for all 3 days was 2.925797 W/m 2 K, 2.9268 W/ m 2 K and 6.214818 W/m 2 K, with the average values being 2.686614 W/m 2 K, 2.641311 W/m 2 K and 3.134446 W/m 2 K for the duration of the experiment. The convective heat transfer coefficients for different food products like potato, pepper, carrot, pear, onion, bean, Albanian pepper and eggplant varied from 0.25 to 3.29 W/m 2 °C (Togrul 2003). Similar convective heat transfer coefficient results were observed by potato, bitter gourd and dates Kumar 2017, 2019;Sansaniwal et al. 2022). The values for heat transfer coefficient remain stable from the 2nd to 6th hour of drying under natural convection due to the effect of stable drying air temperature in the dryer. Figure 13 reports the values of the heat transfer coefficient of ivy gourd under forced convection drying. The average heat transfer coefficient values were obtained as 5.114577 W/m 2 K, 6.700937 W/m 2 K and 8.174063 W/ m 2 K and the maximum heat transfer coefficient values were obtained as 5.176405 W/m 2 K, 6.869181 W/m 2 K and 8.304574 W/m 2 K for the duration of the experiment. The convective heat transfer coefficients of different crops, namely, peas, chillies, onion, cauliflower and potato, were addressed to vary from sample to sample varied, from 3.5 to 26 W/m 2 °C (Anwar and Tiwari 2001b). It was observed that the value of the convective heat transfer coefficient increased with the increase in air velocity in the forced convection dryer due to the uniform air circulation inside the dryer (Akpinar and Toraman 2016). Thus, it was concluded that the value of convective heat transfer coefficients increases with the increase in mass of the ivy gourd, which follows existing literature (Anil Kumar 2007).
From It was observed that the mass transfer coefficient remains constant from the 2nd to 6th hour of drying for all three trials. The rate of moisture removal increases initially, then remains stable and reduces towards the 7th hour of the experiment. Figure 15 shows the average and maximum values for ivy gourd samples under forced convection drying for all 3 days of the experiment. It is obtained as 0.004809 m/s and 0.004854 m/s for day 1, 0.006325 m/s and 0.006442 m/s for day 2 and 0.007602 m/s and 0.00777 m/s for day 3 of the experiment. It shows that the drying air velocity great influences the mass transfer of the samples under forced convection drying. It was also noted that the high value of heat and mass transfer coefficient suggests that the diffusion of moisture was caused due to internal mass transfer resistances and that the resistances had a crucial effect on the entire drying process (Zielinska and Markowski 2007). Figure 16 shows the evaporative heat transfer coefficient variation with drying time for samples dried under natural convection drying.  Sahdev et al. (2017b) and Shimpy et al. (2022a, b) for drying groundnuts. The average value of evaporative heat  transfer was higher from 1 to 1.5 m/s, but the value greatly reduced at 2 m/s velocities. The evaporative heat transfer coefficient rate is higher than the convective heat transfer coefficient for the samples (Kumar et al. 2012). The value of evaporative heat transfer increases significantly with an increase in operating drying air temperature (Kumar et al. 2015). The moisture removal rate was faster initially and reduced after the 5th hour of drying until the end of the experiment (Sahdev et al. 2017a). There was no linear pattern of change that has been observed in the sampling process. This research work would be useful in designing a dryer for obtaining high-quality dried ivy gourd. It will also be helpful in obtaining an optimum moisture level of ivy gourd for retaining its quality during the storage period. The obtained value of convective heat and mass transfer coefficient of drying ivy gourd was within the range of convective heat and mass transfer coefficients of fruit and vegetables reported by Krokida et al. (2002). The measured error for global solar radiation, moisture ratio and temperature were ± 5 W/m 2 , ± 0.002% and ± 0.044 °C in Table 2. The computed values of h c , h m , h e , C and n for ivy gourd natural and forced solar drying were tabulated in Tables 3 and 4.

Conclusion
The experiments were conducted under clear weather conditions for 3 days for ivy gourd under forced, natural convection drying and open sun drying process. The equilibrium moisture content was observed earlier under forced convection with a time of about 7 h compared to 9 h under natural convection drying and about 12 h under the open sun drying process for ivy gourd samples, respectively. Three velocities of 1, 1.5 and 2 m/s were observed for 3 days of forced convection drying, which concluded that higher velocity enables a better drying rate of the samples. The values of heat transfer coefficient for both samples remain fairly constant throughout the drying process for forced convection drying, whereas a lot of fluctuations were noticed for heat transfer coefficients under the natural convection process depending on the drying air temperature and velocity of air inside the dryer. The heat transfer coefficients for forced convection increase with the velocity of drying air inside the dryer. The mass transfer coefficient increased with the increase in drying air velocity for ivy gourd samples, respectively. The evaporative heat transfer coefficient for samples under natural convection fluctuated with time, whereas for ivy gourd samples, there was no pattern in the change of evaporative heat transfer coefficient during the entire process. The convective heat and mass transfer coefficient and evaporative heat transfer coefficient enormously helped to optimize the efficiency of the dryer, drying rate and overall energy consumption. It also helps to identify the most appropriate operating conditions. The future scope of the study is to analyse the drying characteristics of ivy gourd in different pretreatment solutions.