Heinrich Manz

Numerical simulation of heat transfer by natural convection in cavities of facade elements

Abstract Heat transfer by the natural convection of air layers within vertical, rectangular cavities with aspect ratios (A) of 20, 40 and 80 was investigated in relation to applications in building facade elements, such as insulating glazing units, double-skin facades, doors, etc. using a computational fluid dynamics (CFD) code. Boundary conditions were assumed to be isothermal hot and cold wall, and zero heat flux at bottom and top cavity surfaces. Rayleigh numbers were between 1000 and 106, i.e. flow was either laminar or turbulent, and a conduction, transition or boundary layer regime was applied. The study focuses on overall convective heat flow through the air layer. The average Nusselt numbers calculated as a function of Rayleigh numbers are compared with five correlations from the literature which are based mainly on experimental data. Except for one correlation, no calculated Nusselt number deviates more than 20% from the correlations. Deviation is even

Performance of single room ventilation units with recuperative or regenerative heat recovery

Abstract Mechanical ventilation systems with heat recovery for low energy residential buildings have gained an increasing interest during the last few years. This article focuses on single room ventilation units which do not need any ducting within the dwelling. Therefore, they are potentially very suitable for retrofitting use. The performance criteria of these units are similar to those of central systems. A recuperative and a regenerative ventilation unit were investigated by means of experiments and numerical simulations with reference to ventilation efficiency, thermal comfort, heat recovery, electric energy input and acoustics. It was shown that using these units, rooms can be efficiently ventilated (typical air change efficiency 0.6) at a good level of thermal comfort. Temperature efficiencies of up to 0.78 were found at low levels of electric energy input. The main difficulty in the design of these small units is to reach the target values of indoor sound pressure levels ( 20 dB rsp. >30 dB). With reference to acoustic properties, further development of these units seem necessary.

Impact of air leakages and short circuits in ventilation units with heat recovery on ventilation efficiency and energy requirements for heating

The impact of unintentional air flows on the performance of ventilation units with heat recovery is discussed on the basis of single room ventilation units. Assuming an external short circuit (outdoor) and internal (inside the ventilation unit) air leakages, which lead to internal short circuits, a model is developed and characteristic numbers for ventilation efficiency, efficiency of heating load reduction and effectiveness of electrical energy use are derived. Differences between supply and extract air flow rates, resulting in increased air flows through cracks in the building envelope, are taken into consideration too. The use of tracer gas techniques to measure air leakage rates from ventilation units is described briefly. It is shown by numerical examples that unintentional air flows can considerably reduce the performance of ventilation units in terms of ventilation efficiency and, in combination with unintentional heat flows through the casing, energy savings. Therefore, these flows should be avoided or at least reduced to an acceptable level by an appropriate construction, manufacturing process and installation of the units.

Experimental and numerical study of a duct/heat exchanger unit for building ventilation

A mechanical building ventilation unit is presented that unites two functions: fluid transport and heat recovery. Aluminum fins in the supply air duct and in the adjacent extract air duct increase the heat flow from fluid to fluid. This unit is intended to be used mainly for residential ventilation and was investigated by means of experiments and simulations. Airflow rates, temperatures, air humidities and pressure differences were measured on a prototype device in an experimental set-up. Additionally, using a computer program for two-dimensional heat conduction analysis and a simple analytical model, the efficiency of heat recovery was calculated and compared with measurements. Afterwards, the influence of variations of the heat exchanger geometry on thermal efficiency was investigated by means of simulations. It was shown that using this concept it is possible to realize a ventilation unit with high-efficiency heat exchange, e.g. temperature efficiency of 0.7 at a duct length of 6 m.

An empirical validation of modelling solar gains through a glazing unit using building energy simulation programs

Empirical validation of building energy simulation tools is an important component in assessing the reliability of the simulation software. An experiment performed in conjunction with the International Energy Agencys Task 34/Annex 43 was used to assess the performance of four building energy simulation codes used to model an outdoor test cell with a glazing unit. The experiment was run for a 20-day period during October 2004, and experimental cooling powers were compared with predictions from (1) EnergyPlus, (2) DOE-2.1E, (3) TRNSYS-TUD, and (4) ESP-r. Detailed code inputs for optical and thermophysical properties as well as the impact of thermal bridges were quantified through experiments and simulations; numerous statistical parameters and sensitivity analyses were implemented to facilitate a thorough comparison of predicted and experimental cooling powers. The mean percentage differences for all four codes were: 1.9% for EnergyPlus, −3.6% for DOE-2.1E, −6.2% for TRNSYS-TUD, and 3.1% for ESP-r. The implications of various modeling procedures as well as a detailed discussion of the results are provided, specifically concerning the sensitivity of the code cooling power predictions to the selection of convective heat transfer coefficients and algorithms.