Effect of Surface Renewal on the Drop Size Distribution in Dropwise Condensation within a Hybrid Surface

The description of liquid drop growth and drop distribution are two key models in evaluating the thermal performance of dropwise condensation (DWC) heat transfer. The drop size distribution describes the growth process of small drops by direct condensation and large drops by coalescence. The present work investigates the effect of surface renewal and coalescence intensities of DWC within a hybrid surface. Additionally, it examines the validity of the current empirical expression of the drop size distribution that is developed for DWC without considering surface renewal and coalescence intensities. The simulation work illustrates the drop growth process and surface renewal as drops depart and merge with neighboring film regions. The simulation results show that in hybrid DWC, the area fraction occupied by drops (f) lies between 0.28 to 0.296 for the ratio of maximum drop diameter to DWC region width (RD) from 0.125 to 1 and a total temperature drop (∆T) of 2, 4, and 8 ℃. Thus, the drop population is less sensitive to RD, and an average f of 0.288 is generalized. On the other hand, the surface renewal for DWC within the hybrid surface shows improvement for RD > 0.5 with the highest enhancement of 64 to 85% taking place at RD = 1, mainly due to the merging effect. In addition, results for drop size distribution profiles of DWC within the hybrid surface are characterized by a lower population of large drops and a higher population of small drops than full DWC. Additionally, the constant exponent (n) in the literature’s empirical expression is replaced by a polynomial series as a function of drop effective and maximum radii. The impact of surface renewal on coalescence intensity is presented in a relatively steeper slope on the logarithmic scale

Effect of Thermal Bridges of Different External Wall Types on the Thermal Performance of Residential Building Envelope in a Hot Climate

In this paper, the thermal performance of residential building envelopes including thermal bridges (TBs) in a hot climate, using four different exterior wall types, is modelled and assessed. TBs at the junctions between columns and walls and between walls and slabs of the ground floor, roof, and intermediate floors are considered. The tested wall types are classical (two layers of cement blocks with insulation in between), autoclaved aerated concrete bearing (AAC-B), AAC column and beam (AAC-CB), and exterior insulation and finish system (EIFS). The results indicated that thermal bridges have a considerable effect and determine the best external wall type which was the EIFS that has a continuous exterior insulation. EIFS proved to reduce the heat transmission with the outdoor environment for residential buildings by 101.8, 51.2, and 13.9% than the AAC-CB, AAC-B, and classical walls, respectively. Thermal bridges effect on the building envelope using the EIFS is insignificant as the thermal resistance of the envelope and wall differs by less than 1% for small areas. The overall heat transfer coefficients for small buildings are larger than those for large buildings by 8–26%. As the number of intermediate floors increases from 1 to 50, the envelope overall heat transfer coefficient increases by 4.5% for the EIFS, 14.1% for classical, and 19.5% for AAC-CB walls. The AAC-CB, as the common practice wall structure in many hot climate countries, has the lowest performance among the tested wall types

Energy Assessment of the Thermal Bridging Effects on Different Structural Envelope Types Using Mixed-Equivalent-Wall Method

In this paper, the effect of house envelopes including thermal bridges on the daily, monthly, and annual consumption of the air conditioning system of a detached house and an attached house, with a façade in the east, west, north, or south direction, is investigated; moreover, the capacity of the air conditioning system is calculated for detached and attached houses based on the maximum hourly peak load during severe weather conditions. The four tested house envelopes are exterior insulation and finish system (EIFS), autoclaved aerated concrete block (AAC-B), classical (cement blocks with insulation in between), and AAC column and beam (AAC-CB). The work is conducted using a method that combines the finite element method (COMSOL Multiphysics), building simulation (EnergyPlus), and the Engineering Equation Solver (EES) programs. The results indicated that the annual consumption of the air conditioning system using AAC-B, classical, and AAC-CB envelopes is larger than that of EIFS by about 3.74, 11.53, and 20.70% for the detached house, and 1.8, 2.9%, and 6.7% for the attached house, respectively. The annual consumption of the air conditioner of the detached house is larger than the average consumption of the attached house by about 25.3, 27.7, 35.8, and 41.7% for EIFS, AAC-B, classical, and AAC-CB house envelopes, respectively. Using the different façade directions of the attached house, the average effect of the house envelope type on the air conditioning system capacity is about 8.84%, with a standard deviation of 0.466%

Effect of Thermal Bridges of Different External Wall Types on the Thermal Performance of Residential Building Envelope in a Hot Climate

In this paper, the thermal performance of residential building envelopes including thermal bridges (TBs) in a hot climate, using four different exterior wall types, is modelled and assessed. TBs at the junctions between columns and walls and between walls and slabs of the ground floor, roof, and intermediate floors are considered. The tested wall types are classical (two layers of cement blocks with insulation in between), autoclaved aerated concrete bearing (AAC-B), AAC column and beam (AAC-CB), and exterior insulation and finish system (EIFS). The results indicated that thermal bridges have a considerable effect and determine the best external wall type which was the EIFS that has a continuous exterior insulation. EIFS proved to reduce the heat transmission with the outdoor environment for residential buildings by 101.8, 51.2, and 13.9% than the AAC-CB, AAC-B, and classical walls, respectively. Thermal bridges effect on the building envelope using the EIFS is insignificant as the thermal resistance of the envelope and wall differs by less than 1% for small areas. The overall heat transfer coefficients for small buildings are larger than those for large buildings by 8–26%. As the number of intermediate floors increases from 1 to 50, the envelope overall heat transfer coefficient increases by 4.5% for the EIFS, 14.1% for classical, and 19.5% for AAC-CB walls. The AAC-CB, as the common practice wall structure in many hot climate countries, has the lowest performance among the tested wall types

Correlations of Heat Transfer and Fluid Flow Data for Lattice Brick Settings in Tunnel Kilns

This paper proposes correlation equations of heat transfer and pressure drop for the design and operation of tunnel kilns loaded with lattice brick settings of different geometrical parameters in the form of Nusselt number and friction factor. The developed correlation equations considered parameters that were not investigated in previous studies, such as the relative roughness of the bricks and the stack channels, and they also extended the Reynolds numbers to a practical range that was not covered before in a simple, practical form. The correlation equations are valid for Reynolds number between 125 and 10,200, Prandtl number between 0.68 and 0.73, brick’s relative roughness between 0.23 and 0.93, voidage fraction between 0.48 and 0.653, and the geometrical parameters of the tested lattice brick settings. The achieved correlations of the Nusselt number and the friction factor are well compared with the available correlations in the literature in their valid range of parameters. It is found that Nusselt numbers and the friction factors for low-density are higher than those of high-density settings for all considered parameters except the voidage fraction. The effect of the considered parameters confirms that Nusselt numbers increase and the friction factors decrease substantially with the Reynolds number and slightly with the Prandtl number. At a constant Reynolds number, both the Nusselt number and the friction factor increase as the brick’s relative roughness is increased. Moreover, as the stack channel spacing is increased, the Nusselt number decreases, and the friction factor increases. The voidage fraction of the setting has a monotonic effect on both Nusselt numbers and friction factors. Nusselt numbers for high-density are higher than those for low-density settings as the voidage fraction varies