Experimental and theoretical investigations of special type coil heat exchanger with the nanofluid buffer layer

The paper presents the results of experimental and theoretical investigations of special type of coil heat exchanger. The tested device is equipped with three vertical coils and the temperature stratification system. Water is a heating medium in two coils. The refrigerant transferring the waste heat from air conditioning system is the heating medium in the third coil. The finned pipe of this coil has a double wall in which the annular buffer layer with nanofluid is mounted. Thermophysical properties of the applied water based Cu nanofluid cause the enhancement of heat transfer through the buffer layer. The paper presents thermal characteristics of the exchanger received on the basis of measurements performed on the industrial test stand. Measurements were conducted during the operation of the coil with refrigerant. Heat loss to the surroundings, distributions of water temperature in the storage tank, changes of water temperature in time and thermal power of the coil heat exchanger were obtained. The measurement results were compared with those received on the basis of theoretical analysis of the exchanger.


Introduction
Heat exchangers with helical coils have widespread domestic and industrial applications.They may be used in domestic hot water systems, air conditioning systems, refrigerators, chemical and nuclear reactors, steam generators, etc. Applied in heat exchangers the helical coils are often used since they can accommodate a large heat transfer area in compact space and heat transfer coefficients have high values.In recent time coil heat exchangers were the subject of many researchers.The exemplary works on the considered exchangers are [1][2][3][4][5].Prabhanjan et al. presents in [1] the comparison of heat transfer between coil heat exchanger and straight tube heat exchanger.The work [2] by Zachar deals to improvement of heat transfer processes in coil heat exchanger with spirally corrugated wall.In the paper [3] Genic et al. presents the results of investigations of the influence of geometry parameters of coil heat exchangers on heat transfer coefficients.Urbanowicz and Wojtkowiak presents in [4] a comparative analysis of correlations for heat transfer at surfaces of coil and suggests an original method of calculation of heat exchanger with helical coil.In turn, characteristics of agitated helical coil heat exchanger operating with the nanofluid are described by Srinivas and Vinod in [5].Tested in the present work the coil heat exchanger works with the nanofluid too.The investigated device is designed to warm domestic hot water.It is equipped with three vertical coils immersed in storage tank filled with heated water.The orientation of the coil system is shown in Fig. 1.Additionally the temperature stratification system is applied in the storage tank.Water is a heating medium in two coils.The refrigerant transferring the waste heat from air conditioning system is the heating medium in the third coil.The finned pipe of this coil has a double wall in which the annular buffer layer with nanofluid is mounted.According to the rules the layer have to protect against the possible refrigerant leakage to the hot domestic water tank.The gap is filled by the nanofluid consisting of distilled water and coper nanoparticles.Thermophysical properties of the applied water based Cu nanofluid cause the enhancement of heat transfer through the buffer layer.Thus, the buffer layer with nanofluid acts as a protection, and in the other hand it improves thermal characteristics of the exchanger.A detailed description of the idea and the analytical model of the proposed coil heat exchanger is contained in [6].In turn the results of calculations of thermal power of water coil of the exchanger considered are included [7].This paper presents also the experimental results of water coil in comparison with received theoretical results.The present work contains the results of thermal measurements which were conducted during the operation of the coil with refrigerant.Heat loss to the surroundings, distributions of water temperature in the storage tank, changes of water temperature in time and thermal power of the coil heat exchanger were obtained.The measurement results were compared with those received on the basis of theoretical analysis of the exchanger.Experimental investigations on considered heat exchanger with the coil that uses the waste heat from air conditioning system were performed on industrial test stand.The detailed description including the technological scheme of the stand, the specification of applied elements and the research possibilities of the stand were presented in [8].The simplified scheme of the experimental stand is shown in Fig. 2. The measuring system of the experimental stand has enabled the measurements of temperature of the outer surface of the tank and the same determination of mean temperature of domestic hot water in the storage tank Tm was possible.Heat conduction through the wall of the storage tank has been omitted.Tm has been obtained with the use of eight resistance temperature detectors Ti [8] located on the surface of the tank (see Fig. 3) according to the formula

Experimental investigations
where Vi -control volume assumed for the temperature measured Ti.

The basic geometrical parameters necessary to calculate
Tm is summarised in Fig. 3.The results of investigations of the heating process of water in the storage tank are presented in Fig. 4.There are changes of water temperatures T1 -T8 during the thermal start-up of the exchanger operating with the use of coil with refrigerant -the exchanger of heat recovery system.The refrigerant R407C has been applied.
where: V-volume of water in the tank, ρw, cw -density and heat capacity of water, Q & -thermal power of the exchanger, loss Q & -heat loss to the surroundings.
On the basis of eq.( 2) the thermal power of coil exchanger of heat recovery system has been calculated.Heat losses from the storage tank have been measured with the use of three film heat flux sensors [8].The results of the measurements and the linear approximation are presented in Fig. 6.The product ρwcw in eq.( 2) has been calculated on the basis of approximation of literature data [9] according to the formula The obtained mean value of thermal power of coil heat exchanger operating with refrigerant as a heating medium is equal to 2.452 kW.

Theoretical calculations
Theoretical calculations of the coil heat exchanger were based on heat transfer processes occurring in the exchanger during operation.The analysis has related to: convective and radiative heat transfer from the outer surface of the storage tank, convective heat transfer between water and the inner surface of the tank, natural convection from the outer surface of finned coil pipe, heat conduction through the coil wall with taking into consideration the additional thermal resistance of the nanofluid buffer layer and convective turbulent flow of the refrigerant inside the coil pipe.The main formulas used to calculations are summarised in Table 1.Characteristic dimensions occurring in dimensionless numbers were described in [6].The presented procedure has enabled obtain heat losses from the storage tank and the thermal power of the coil heat exchanger.The thermal power of coil exchanger of heat recovery system -the coil with refrigerant -has been calculated with the use of the general formula where: A -overall coil pipe surface; U -overall heat transfer coefficient; T1, T2 -temperature of refrigerant at the inlet and the outlet of the coil, respectively, Tmassumed mean temperature of water.The overall heat transfer coefficient takes into account the following components: convective heat transfer inside coil, conduction through the inner wall of the coil pipe, heat transfer in the nanofluid buffer layer, conduction through the outer wall, convective heat transfer from finned wall of the coil.It is given by where: hi -heat transfer coefficient at the inner surface of the coil pipe Aip, heq -equivalent heat transfer coefficient of the finned surface, Ain_f and Af -intercostal surface and the surface of the fin respectively, ηfefficiency of the fin, Nf -the number of fins.Rk1conduction thermal resistance of the inner coil pipe; Rk2conduction thermal resistance of the outer finned coil pipe; Rb -conduction thermal resistance of the buffer layer taking into account equivalent thermal conductivity of the nanofluid.Calculations were performed with taking into account the temperature dependence of thermophysical properties of the applied medium.The latent heat of the refrigerant was omitted because of temperature range of the working medium.Since heat transfer coefficients were implicit temperature functions, the system of nonlinear equations was built.It has been solved using the secant method.Geometrical parameters of the tank and the coil were assumed as the same as in the real object.The main data are summarised in Table 2 according to the presented in Fig. 7 the scheme of the double wall finned pipe.Table 2 also presents the main characteristic of the nanofluid applied in the buffer layer.The results of calculations of thermal power of the exchanger at assumed temperatures of the refrigerant in the coil are summarized in Table 3.

Conclusions
Experimental and theoretical investigations on a special type of coil heat exchanger have allowed to formulate some conclusions from the obtained results.The fast process of water heating in the storage tank was observed.By the time 5 minutes after the start of the exchanger the temperature of water is not rising to the 60% of the relative height of the tank.After 1900 second from the start of water heating with the use of coil with refrigerant the difference between T8 and T3 (see Fig. 4) is only 1.6 o C. Hot water is transported to the upper zone of the tank by a temperature stratification system.The obtained results indicate good operating of the system and may be helpful in optimization of the stratification system.The calculation results of heat loss from the storage tank are in good agreement with measured heat loss.The linear approximation of the calculated heat loss is in the area of the scatter of measurement results.Mean value of heat loss measured at maximum water temperature 32 o C is equal to 64.1W while the calculated value is 57.3W.The relative difference is about 10%.Good compatibility is also between the measured and calculated thermal power of the exchanger.Experimentally obtained thermal power is 2.452 kW at measured supply temperature of refrigerant 44.74 o C. Calculated values are respectively equal: 2.7 kW at 45 o C. The results obtained indicate a valid model for theoretical calculations.It can be applied to appropriate selection of insulation of the storage tank to reduce heat losses.Also it can be used for modeling of heat transfer in the buffer layer with nanofluid.

Fig. 1 .
Fig. 1.System of coils applied in the heat exchanger

Fig. 3 .
Fig. 3. Location of temperature detectors and relative control volumes of the storage tank

Fig. 4 .Fig. 5 .
Fig. 4. Temperature of water in storage tank in function of time of water heating

Fig. 6 .
Fig. 6.Heat losses from the outer surface of the storage tank diameter of the coil pipe D -characteristic dimension of the coil hr -radiative heat transfer coefficient k, keq -thermal conductivity and equivalent thermal conductivity Nu -Nusselt number Pr -Prandtl number Ra -Rayleigh number Re -Reynolds number Twall, Ta-wall and ambient temperature ɛ -surface emissivity σ -Boltzmann constant

Fig. 7 .
Fig. 7. Scheme of the double wall finned pipe

Table 1 .
Equations for the calculation of heat transfer process in the coil heat exchanger[10 -12]

Table 2 .
Geometry of the finned copper pipe of the coil with refrigerant and calculated thermal power of the exchanger

Table 3 .
The results of thermal power calculations

Table 4 .
Heat losses from the storage tank at Ta = 26.4o C, V = 200dm 3 Calculations were made for uninsulated tank (real conditions) at assumption ambient temperature 26.4 o C. The linear approximation of the calculation results is shown in Fig.5.