**Nomenclature**

**3. Recommendations for future studies**

triangular cross sections such as right isosceles triangular.

atmospheric life time of R245fa (7.6 years).

teristics of flow condensation.

nanorefrigerants.

pure.

microscales can be done using:

90 Heat Transfer Studies and Applications

Finally, recommendations for future studies will be given. These new points can be expected to be the research focus in the coming years. Studying the condensation pressure drop in

**1.** The experiments with new kinds of non-circular shapes like trapezoidal, elliptical,.., etc. To the best of the authors' knowledge, the study of condensation pressure drop in microscales of elliptic cross-section is not yet tackled in literature. Only recently a new interest has been devoted to the elliptical cross-section, produced by mechanical fabrica‐ tion in metallic microchannels for practical applications in MEMS. Also, the experimental study of the condensation pressure drop in microscales can be done with using various

**2.** The experiments with new kinds of environmentally friendly refrigerants such as HDR-14, which is low global warming fluid for replacement of R245fa. The GWP is an index used to compare the potential of gases to produce a greenhouse effect and the reference is CO2 with a value of 1. HDR-14 has a global warming potential (GWP) of only 7, much lower than the value of 930 of R245fa (both considering a period time horizon of 100 years). Also, HDR-14 has a much lower atmospheric life time (0.1 year) in comparison with the

**3.** The experiments with oil in the refrigerant loop (refrigerant/lubricant mixture) at various concentrations. For example, Akhavan-Behabadi et al. [88] utilized polyolester oil (POE) as the lubricant in R600a/POE mixture to study experimentally the heat transfer charac‐

**4.** The experiments with new types of working fluids such as refrigerant/lubricant/nano‐ particles mixture. For example, Akhavan-Behabadi et al. [88] used R600a/ polyolester oil (POE)/CuO nano-refrigerant to study experimentally the heat transfer characteristics of flow condensation. Nano-refrigerant is a type of nanofluid where a refrigerant is used as the base fluid [89]. Recently, Dalkiliç and co-workers [90, 91] presented a review paper on

**5.** The experiments with new types of refrigerant blends. For instance, new refrigerant blends, like R-417A, are becoming very important due to the possibility of using them in R-22 systems with only minor changes (drop-in refrigerants) [92]. R-417A is the compo‐ sition with mass fractions of 46.6% R-125, 50% R-134a, and 3.4% R-600. Also, we can carry out the experiments with new kinds of refrigerant blends like R1234yf and R1234ze(E) as constituents as well as blends of old refrigerants with Hydro-Fluoro-Olefin (HFO) refrigerants. These blends can be azeotropic blends or zeotropic blends like zeotropic mixture R32/R1234ze(E). R32 (CH2F2) is flammable and has for this reason not been used

**6.** Similar to recent work on condensation in macroscales at microgravity conditions [93, 94], studies on condensation pressure drop in microscales at microgravity conditions can


σ; surface tension, N/m ψ; dimensionless surface tension term (-) Subscripts bubble; bubble cond; condenser cr; critical d; diameter eq; equivalent g; gas gc ; gas core go ; gas phase with total mass flow rate h ; hydraulic i; inner in; inlet l; liquid lo ; liquid phase with total mass flow rate m; mean sat; saturation slug ; slug transition transition tube; tube UC; unit cell unit cell unit cell tp; two-phase

*E* ; parameter, Eq. (28) *Eu* ; Euler number (-)

92 Heat Transfer Studies and Applications

*F* ; parameter, Eq. (19) *F* ; parameter, Eq. (29) *G* ; mass flux, kg/(m2

*H* ; parameter, Eq. (20) *H* ; parameter, Eq. (30)

*L;* channel length, m

*P*; pressure, Pa

*N*; total number of unit cells

Re ; Reynolds number (-)

Su; Suratman number (-)

W ; parameter, Eq. (17) We ; Weber number (-)

T ; temperature, o

U ; velocity, m/s

x ; mass quality (-)

Greek Symbols

∆; difference

ϕ*2*

α; void fraction (-)

ρ; density, kg/m<sup>3</sup>

Z ; parameter, Eq. (18)

*f* ; Fanning friction factor (-)

.s)

*Jg* ; dimensionless gas velocity = *xG*/[*gd*h*ρ*g(*ρ*<sup>l</sup>

RR ; relative roughness of the tube, Eq. (26)

C

X ; Lockhart-Martinelli parameter (-)

; two-phase frictional multiplier (-)

μ; dynamic viscosity, kg/m.s

*-ρ*g)]0.5

*Ra* ; arithmetical mean deviation of the assessed profile (according to ISO 4287: 1997), *μ*m

*Rz* ; maximum height of profile (according to ISO 4287: 1997), μm
