DETERMINATION OF HEAT TRANSFER COEFFICIENTS UNDER

CLIMATIC CONDITIONS OF ISTANBUL

Mustafa Kemal Kaymak and Ahmet Duran Şahin

İstanbul Technical University, Aerospace and Aeronautics Faculty, Meteorology Department, Energy Group, 80626 Maslak/İstanbul

mustafakaymak@gmail.com, sahind@itu.edu.tr

Atmospheric Conditions and Natural Gas Consumption

• Main natural gas consumption amount occurs in winter.

• Consuming natural gas or other fossil/ renewable sources for heating means to fight against atmospheric conditions.

• Generally atmospheric temperature is taken the main variable to effect natural gas consumption

• But some of others are forgotten for example wind

Main Aim• In this research cooling effect of wind speed to buildings is considered under

climatic conditions of İstanbul in detail. Firstly, external surface heat transfer coefficient is calculated depends on wind velocity changes.

• As known that depend on seasonal variation wind speed effects also changes. In other words in winter time wind speed values cause to high level heat losses and so additional heating required.

• In contrast to this, in summer time wind speed values cause to additional cooling and could be used as supporter for air conditioning of buildings. Depend on these conditions cooling and heating estimations should be considered for energy planning.

Wind chill and Its Importance• The Wind Chill Index is based on an equation first proposed in 1939 by

Paul Siple, a famous geographer, polar explorer, and an authority on the Antarctic. In the 1940s, he and fellow Antarctic explorer Charles F. Passel conducted experiments on the amount of time it took for water to freeze in a plastic cylinder while exposed to the elements. They discovered that the time it took for the water to freeze depended on the initial temperature of the water, the outside air temperature, and the speed of the wind.

• Moving air carries heat away from the body more effectively than air that is not moving. If there is no wind, the heat radiating from a person's body will stay near the body and warm the air around it. Therefore, the wind chill is simply a means of describing the effect of the movement of air on the heat loss of a body.

• The wind chill factor or equivalent temperature uses a neutral skin temperature of 33 °C as a baseline value.

Heat transfer coefficient and Wind chill index related with human body

• Knowing the latent heat of fusion of water, the surface area of the cylinder, and the air temperature, they are able to calculate heat transfer coefficients, which is called as wind chill factors, hwc. This factor is related to the wind speed, V (m/s), by an emprical equation

• The wind chill index (WCI), which is an estimate of heat transfer, is obtained by multiplying the wind chill factor (hwc) by the difference between an assumed skin temperature (33°C) and air temperature (Tair)

VVhwc 1045.10

)33( airwc TChWCI

Heat Losses and Structures• As known that buildings could be considered as complex structures

for heat transfer that continuously varies with weather conditions, flow field and surface geometries.

• It is well known that climatic comfort sensation in a room is not only dependent on the indoor air temperature, but also on the inner and outer surface temperatures.

• Under passive heating or cooling conditions, indoor air and surface temperatures change with the rate of heat flow through the building envelope is the function of the thermo physical and solar radiation properties of the opaque and transparent parts.

• Airflow around buildings affects weather and pollution protection at inlets, and the ability to control environmental factors of temperature, humidity, air motion, and contaminants.

• Wind causes surface pressures that vary, around buildings, changing intake and exhaust system flow rates, natural ventilation, infiltration and exfiltration, and interior pressure. The mean flow patterns and turbulence of wind passing over a building can cause a recirculation of exhaust gases to air intakes.

• If a building is oriented perpendicular to the wind, it can be considered as consisting of several independent rectangular blocks.

• As known that topographical conditions are very important for calculation of wind profile at different heights. Roughness length represents sea, shore, land/complex topography and urban areas effects to the wind profile. For urban area roughness length is suggested to consider as 1.5 m or over this value.

• The mean speed of wind approaching a building nonlinearly increases with height above the ground. Both the upwind velocity profile shape and its turbulence level strongly influence flow patterns and surface pressures.

DATA AND STUDY AREA• In order to evaluate and assess heat losses depend on wind speed

values, a database is considered of hourly average wind speed and direction measurements taken between 1996 to 2006 in the northern part of Istanbul, which is located between 40.97E longitude and 29.08N latitude.

• This area comes under the influence of the mild Mediterranean climate during the summer, and consequently experiences dry and hot spells for four to five months, with comparatively little rainfall.

• During the wintertime, this region comes under the influence of high-pressure systems from Siberia and the Balkan Peninsula and low-pressure systems from Iceland. Hence, mainly northeasterly or southerly winds influence the study area, which also has high rainfall amounts in addition to snow every year in winter. Air masses originating over the Black Sea also reach the study area.

Figure Wind rose for considered region

0

2000

4000

6000

8000

10000

12000N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

APPLICATION• As known that wind profile in other words wind speed changes with

height depend on topographical conditions. These conditions are summarized with roughness length that represents surface friction and topographical complexity.

• Roughness length changes between 0.00001 m to 2.5 m from sea level to very high complex urban areas respectively.

• In this paper, depends on complex urban conditions roughness length (z0) is considered as 1.5 m for wind profile estimation. It should be remembered that wind speed values (from reference height to desired hub height) show higher changes at complex conditions than sea level.

• Wind speed profile at desired height could be estimated by using logarithmic approximation as

)/ln(

)/ln(

01

0212 zZ

zZVV

In here, V1 represents measured wind speed values at 10 m that is reference level for this paper and V2 represents wind speed at desired building height.

• In addition to this, as mentioned before roughness length changes from sea level to complex urban topographical conditions.

• In this paper wind speed profile is evaluated in complex urban conditions at different heights which are considered 10m, 30m, 60m, 90m, 120m, 150m, 180m and 210m.

• It is estimated that in the afternoon wind speed values reach to the maximum value and considering these data there is a suddenly jump between 10 m and 30 m heights after this level variation is getting stable.

• In order to satisfy the minimisation of supplementary mechanical energy demand, the optimum value of the overall heat transfer coefficient for opaque components should be determined in terms of the climatic conditions of the predominant period for the region.

• To select the predominant period, it is necessary to compare the under heated and overheated periods, by taking their duration and severe climatic conditions into account.

Figure Wind profile changes during day for different heights in Sarıyer-İstanbul

• Calculation of external surface heat coefficient, hw at different heights of buildings depends on wind velocity is the main approach of this study.

• The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE), in their most recent revision of the ASHRAE Handbook, recommend a standard value for the external convection coefficient, hw , of 29 Wm-

2K-1 at a wind speed of 6.7 ms-1.

• In this paper, hw values at different heights are estimated based on above equations. It is shown in the next Figure changing of hw and wind velocity with height.

• It is easily seen that there is an increase values of hw with height of building. For windward side of building above 30 m during the day all convection coefficients are higher than standard ASHARE value.

for leeward as

hw = 7.4 + 4.0 V

for the winward side

hw = 4.2 + 3.5 V

• In other words, over 30 m height buildings standard ASHARE value that is 29 W/m2K, should be rearranged in İstanbul conditions. Additionally, if height of building is increased differences between levels will be increased.

Figure Heat loss coefficient variation during day for different heights in Sarıyer-İstanbul on windward side.

For leeward side of the building approximately 25% less than windward heat losses occur. Atmospheric boundary layer conditions also effect low level urban conditions on

the leeward side.

Figure . Heat loss coefficient variation during day for different heights in Sarıyer-İstanbul at leeward side.

Heat transfer coefficient and Wind chill index related with structures

• External surface heat coefficient, hw let us to know wind chill factors for buildings. This factor is related to the wind speed, V (m/s), by an emprical equation for windward of building

• and for leeward of building

• The wind chill index for building (WCB), which is an estimate of heat transfer, could be obtained by multiplying the wind chill factor for buildings (hw) by the difference between an assumed building envelope temperature (14.98°C) and air temperature (Tair)

iiwVh 0.44.7

)98.14(iairiwi TChWCB

iiwVh 5.32.4

n

i

n

iiairiwi TChWCB

1 1

)98.14(

Summation of WCB gives heating or cooling degree of the building.

requireheating

requirecoolingorheatinglessif

0.0

0.0

In short, in İstanbul conditions buildings losses much heat energy than gains

CONCLUSIONS• In this paper external heat transfer coefficient of the building

envelope is considered under climatic conditions of İstanbul.• It is estimated that over 30 m height ASHARE standard heat

convection coefficient for building envelope should be rearranged in İstanbul.

• Additionally, complex topographical conditions effects are decreased above 120 m in the boundary layer of İstanbul.

• For the perspective of heating and cooling due to the wind speed values maximum heat losses occur after midnight hours.

• A new approache related with wind chill index suggested for building heating or cooling requires.

Thank You