**2.1 Studies related to Al2O3**

Till date, researchers developed various methods to improve heat transfer characteristics of refrigerants inside the vertical and horizontal tubes. Adding nanoparticles to base refrigerant is one of the most efficient methods to enhance the thermal characteristics of refrigerant.

**Figure 2.** *Schematic diagram of vapor compression refrigeration cycle.*

#### *Effect of Nanoparticles on Performance Characteristics of Refrigeration Cycle DOI: http://dx.doi.org/10.5772/intechopen.89236*

Jwo et al. [8] dispersed 0.1% mass fraction of Al2O3 particles in polyester oil and reported 2.4% reduction in compressor work. Mahbubul et al. [9, 10] reported significant increment in thermal conductivity and viscosity with Al2O3 nanoparticles dispersed in R141b. Later, the author observed increment in heat transfer coefficient and pressure drop up to 383 and 181%, respectively using particle volume fraction between 1 and 5% with fixed mass flux of 100 kg/m<sup>2</sup> s. Mahbubul et al. [11] reported large frictional pressure drop of R134a/Al2O3 nanorefrigerant as compared to R113/CuO nanorefrigerant flows inside horizontal smooth tube due to higher vapor quality [12].

Sun and Yang [13] studied the effect of Alumina-R141b, Cu-R141b, Al-R141b and CuO-R141b with mass fractions 0.1–0.3 wt% in a computer aided test rig on flow boiling heat transfer in horizontal tube and reported that Cu-R141b nanorefrigerant had the highest heat transfer coefficient as compared to other mixtures. Kedzierski [14] dispersed 1.6% volume fraction of Al2O3 in R134a/POE mixture flows on a horizontal and rough flat surface and found that higher volume fraction of nanoparticles with low average diameter has greater positive effect on heat transfer characteristics of base refrigerant. Further, Kedzierski [15] investigated the effect of Al2O3 nanoparticles on the pool boiling characteristics of R134a/ POE mixture inside the rectangular finned surface and reported that 3.6% nanoparticle volume fraction enhanced the boiling heat transfer performance up to 113%. Tang et al. [16] reported that using δ-Al2O3 with R141b significantly improved pool boiling heat transfer coefficient in the system but adding surfactant at higher concentration corrupted the heat transfer process.

Mahbubul et al. [17] dispersed Al2O3 in R141b refrigerant for thermal conductivity and viscosity investigation. The author reported that viscosity and thermal conductivity of R141b/Al2O3 nanorefrigerant at 2% volume fraction are 179 and 1.626 times greater than pure refrigerant. Jwo et al. [8] used 0.05–2% weight fraction of Al2O3 particles with R134a and R12, respectively and reported that R134a refrigerant replaces R12, as polyester oil replaces mineral oil. Further, 0.1 wt% fraction of nanoparticles in R134a refrigerant reduced the energy consumption by 2.4% which significantly improved the COP of refrigerator. Kumar and Elansezhian [18] experimentally investigated the effect of R134a/Al2O3/PAG blend on the overall performance of VCR cycle and observed lower energy consumption by 10.32%. Author stated that using nanoparticle in refrigeration system is a cost effective method which improve its COP and length of capillary tube is reduced.

Subramani and Prakash [19] observed 25% less energy consumption and 33% overall COP enhancement in VCR cycle using Al2O3 nanorefrigerant. The freezing capacity of the cycle was also improved. Yusof et al. [20] dispersed 0.2% Al2O3 particles in polyester (POE) lubricant and reported 7% improvement in system COP and 2.1% reduction at compressor energy consumption. Cremaschi et al. [21] studies that alumina nanoparticles did not improve the solubility between refrigerant and lubricant, while addition of nanoparticles had slightly lowered the solubility of R410a/POE.

## **2.2 Studies related to CuO**

Kedzierski and Gong [22, 23] observed heat transfer improvement between 50 and 275% using 0.5% mass fraction of CuO particles with R134a/RL68H and R134a/ POE blend. Moreover, R134a/RL68H blend shows higher heat transfer enhancement as compared to the R134a/POE blend. In Later study, the author used 2 Vol% fraction of CuO particles in R134a refrigerant and reported nanorefrigerant has higher heat flux. Bartelt et al. [24] dispersed 0.5–1% mass fraction of CuO nanoparticles in R134a/polyester blend in horizontal flow boiling conditions and

ø ¼

masses of nanoparticles and lubricants respectively. The formation of

understands the contribution of nanorefrigerants and nanolubricants in

nanorefrigerant is possible as shown in **Figure 1**.

*Formation of nanorefrigerant in VCR cycle.*

*Low-temperature Technologies*

**2. Improvement in VCR system performance**

refrigeration cycle.

**Figure 1.**

is shown in **Figure 2**.

**Figure 2.**

**102**

**2.1 Studies related to Al2O3**

thermal characteristics of refrigerant.

*Schematic diagram of vapor compression refrigeration cycle.*

*mp ρp mp ρp* <sup>þ</sup> *mL ρL*

where, ø is the volume fraction in percentage, *ρ<sup>p</sup>* and *ρ<sup>L</sup>* are the density of nanoparticles and density of the lubricant respectively; and *mp* and *mL* are the

The present chapter aims to define the mechanism that steers towards improvement in overall VCR cycle performance using nanolubricants and nanorefrigerants. The authors' hope that this paper will be useful to define the research gaps and

The section consists of four sub-sections. First part concerns research findings related to Al2O3 nanoparticles and the other parts deals with review studies related to CuO, TiO2 and CNT nanoparticles. The schematic diagram of refrigeration cycle

Till date, researchers developed various methods to improve heat transfer char-

nanoparticles to base refrigerant is one of the most efficient methods to enhance the

acteristics of refrigerants inside the vertical and horizontal tubes. Adding

� 100 (1)

found 42–82% and 50–101% heat transfer enhancement for 1 and 2% mass fraction respectively. Peng et al. [25] dispersed 0.1 and 0.5 wt% CuO nanoparticles in R113 refrigerant to study heat transfer performance inside a horizontal rough pipe and reported 29.7% HTC using nanoparticles in base refrigerant. Henderson et al. [26] reported lower heat transfer performance with 0.5 and 0.05 vol% of CuO and SiO2 nanoparticles dispersed in R134a and R134a/POE blend during boiling flow conditions in horizontal tube. Further, the author used 0.02, 0.04 and 0.08 vol% of CuO nanoparticles in R134a/POE blend and observed that nanoparticle with 0.04 and 0.08 vol% improved heat transfer performance up to 52 and 76%, respectively. Kedzierski and Gong [27] dispersed 0.5% mass fraction of CuO nanoparticles in polyester oil and observed 275% improvement in heat transfer with base refrigerant R134a.

Later, Bartelt et al. [28] extended the experiment of Kedzierski and Gong [27] and observed that 2% concentration of CuO nanoparticles gives the highest improvement up to 101% (**Tables 1** and **2**) [29, 46].

Abdel-Hadi et al. [30] experimentally found that CuO nanoparticles with average size 25 nm and concentration 0.55% is an optimum value which significantly enhanced the evaporative heat transfer coefficient. Kumar et al. [29] observed 7% reduction in compressor energy consumption and 46% enhancement in COP with dispersion of 0.2–1 wt% fraction of CuO nanoparticles in compressor lubricant. Moreover, the author reported reduction in friction and wear in compressor using nanoparticles in base lubricant. Peng et al. [31] used Cu nanoparticle in R113/VG68 (ester oil) mixture. It was observed that using Cu nanoparticles with average size of 20 nm strongly improved the heat transfer performance up to 23.8% as compared to other particles sizes of 50 and 80 nm. Akhavan-Behabadi et al. [32] found 83% increment in heat transfer rate with 1.5% mass fraction of CuO nanoparticles dispersed in R600a/polyester oil condensed inside the smooth horizontal tube (**Figures 3** and **4**) [29, 41, 46].
